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Tips for working with fiberglass

Fabricating a custom subwoofer enclosure for the trunk

A Crutchfield employee and installation expert shares his process and some pro tips for working with fiberglass. He documented his recent build of a custom subwoofer enclosure and let us pick his brain for the details. Read below for an intro to custom fiberglass work.

When Darren, Crutchfield's vehicle integration gear guru, overhauled the factory system in his 2017 VW Golf Alltrack, he built a fiberglass enclosure for his 10" Hertz subwoofer. No stranger to working with fiberglass, Darren decided he'd make an enclosure that molded to the shape of the spare tire and be removable if he needed quick access to the spare. It took him nine steps to go from concept to custom box.

Fiberglassing starter kit

You'll find some of the essentials for a project like this in our box-building tools, but most will require a trip to your nearest hardware store. You might also opt to borrow a tool or two from your neighbor if this is your first go at a custom build. And then there's the stuff you might be able to find lying around the house.

Before diving into building his subwoofer enclosure, Darren gathered up the following...

List of tools

  • Measuring tape 
  • Circular saw and jig saw or router
  • One or two disposable containers (You'll need these for applying resin. Old coffee cans, sour cream containers, etc. work best!)
  • Rubber gloves (This project could get messy.)
  • Clothes you don't mind ruining (Like really messy.)
  • Painter's mask (And fume-y.)
  • Paint brush (For applying resin)

List of materials

  • Multiple rolls of painter's tape (You'll need these for taping off the area where your custom-fit box will be.)
  • MDF (If you have any structure framing you need to do, fiberboard is a good, cheap option.)
  • Fiberglass chopped strand mat (This is what gives your enclosure its custom shape.)
  • All-purpose fiberglass resin (Applied to the fiberglass mat, this is what gives your enclosure rigidity.)
  • Sand paper (Never hurts to have sand paper to smooth out any rough edges.)
  • Caulk (For sealing any seams and ensuring an airtight enclosure)
  • Box carpet and 3M spray adhesive (For a finished look)
  • Speaker terminal and speaker wire (For easy hook-up and removal of your enclosure)

When it comes to materials like painter's tape, fiberglass mat, and resin, Darren recommends overestimating how much you'll need. He had to make two trips to the hardware store for this project, and when you're covered in dried resin, the last thing you want to do is go out in public (especially in the car you're currently working on).

Planning and building your box

With all your supplies gathered, you're ready to take on your project. Darren set up shop in his garage because it's temperature-controlled and all his tools are close at hand. If you don't have a garage, you'll want to tackle this job when pleasant weather is on the horizon and in a space that has plenty of ventilation. Fiberglass resin is unbeatable for its rigidity once it dries, but in liquid form, it's noxious stuff.

Darren stretched out the following steps over days, but if you have the time to work 'em into a weekend, go for it!

Prepping your custom sub location

Step 1: Test fit the location

Place the sub in your chosen location to get a sense of how it will mount and if there will be sufficient air space. (You'll usually find your sub's ideal enclosure volume in the speaker's manual.)

Tip: You may want to take some initial measurements and keep them on a Post-it® note nearby for easy reference.

Protecting with painter's tape

Step 2: Prep the location

Completely seal off the area you'll be using to create your mold. In Darren's case, he laid down layer upon layer of painter's masking tape to ensure that nothing could seep through and damage his VW interior. Even though it was a relatively small area, Darren used a couple rolls of tape.

Tip: You can't be too careful. Some folks go as far as adding a layer of aluminum foil as well to ensure nothing leaks through.

Building a frame for the fiberglass

Step 3: Build a skeleton

Confirm the volume you'll need for your enclosure and build accordingly. Darren cut a board to frame out a fourth wall so that every part of the box would be structurally sound. This is the foundation of your box, so make sure it won't move at all as you take on the next step.

Tip: Be sure to overestimate the box volume you'll need. When the time comes to add the top to your box, it's always going to be easier to refine the structure than build it up.

cutting fiberglass mat with scissors

Step 4: Arrange fiberglass mat

Cut your fiberglass mat into manageable strips and pieces. Organize them to cover the surface area of your enclosure. 

Tip: Weight the mat in key locations to keep your strips in place as you begin the next step.

Fiberglass shell

Step 5: Apply resin

Pour over, dab, or paint resin to coat your arranged mat strips and seal any gaps. The more coats of resin the better. You'll want to check the directions on resin packaging for ideal drying temperatures. Darren built his box in the dead of winter, but because he did the job in a temperature-controlled garage, this step went smoothly and didn't drag on.

Tip: Take your time, assess progress, and apply more coats as needed.

Cleaning up your mold

Step 6: Finalize measurements

Once the structure is dry to the touch, remove it from the vehicle and peel off any tape that has adhered to the bottom. With your new box outside the vehicle, you'll have an easier time finishing your work. Confirm that the volume of the box matches your initial plans.

Tip: There's no one correct way to measure the volume of your box. Some fill the enclosure with styrofoam peanuts or water. Darren measured the space in manageable layers. 

Create a top for the enclosure

Step 7: Build and secure your lid

Measure and cut a piece of MDF that will be the lid of your box. Use your sub's listed cutout diameter to create a hole for mounting the speaker. Secure the lid to the molded enclosure.

Tip: Darren caulked the seams of the box just to be sure it was perfectly sealed.

Finished fiberglass subwoofer box

Step 8: Finishing touches

You may want to add speaker terminals for easy hookup to your system. Check out our assortment of other subwoofer box accessories for ideas. Darren used 3M adhesive spray to finish the box with a carpeted exterior, but this is your chance to truly customize your creation. Be bold and give it an exterior that suits your vehicle's interior. Then, wire and secure your sub (or subs) to your new custom enclosure and start thumpin'.

Darren's one final tip

As we wrapped up our conversation with Darren, he summed up with this little nugget of wisdom: "Fiberglassing is really a project for a true audio enthusiast. It's messy work and can be painstaking at times, so before you dive in, it's important to be really excited about what you're going to create."

Keep in mind that every project is different and there's no end to creative solutions. If you're stuck at any point in the process, explore YouTube to find videos of enthusiasts showing off their work and the process. And if you have any advice for others, feel free to share it in the comments of this article. 

  • Dee

    Posted on 4/16/2023

    Great article as usual. Bookmarked for future reference. Question though. Does the fiberglass bond with the MDF? Is that bond strong enough, long term, to handle the vibrations created by the bass? About to build my first fiberglass bottom enclosure and just unsure how strong the bond is.

  • Adrian B. from Nashville

    Posted on 9/30/2021

    Fiberglass & resin are great tools for larger speaker enclosures. Another method for smaller custom enclosures is to use stretchy fabric such as pantyhose, spun cotton, or spandex-type materials with resin. It is a popular way to make custom door panel or A-pillar additions. It's a little trickier and requires a LOT more sanding, but once it is covered with new materials or even gloss paint when super-smooth, it makes a fantastic visual impact.

  • Scott minnick from Radford

    Posted on 6/24/2021

    Can I do this to mold in 4 18s in my station wagon and it handle 4000 watts

    Commenter image

    Alexander H. from Crutchfield

    on 7/16/2021

    Scott, you'll search online for some sort of template (if it exists) but working with fiberglass allows you to customize an enclosure for whatever your needs are.
  • Kevin from Wise

    Posted on 12/13/2020

    Nice job ! I've worked with fiberglass a lot over the years as I repaired a lot of semi truck hoods ! For someone who has never worked with fiberglass watch a lot of U-tube videos before trying this ! Make something real small before jumping into a sub box ! Just my opinion! Messy is not the word ! But this article was extremely helpful! I have a 2013 Subaru Outback that has a factory Sub. Trying to improve my bass I installed a 8 inch Audiofrog sub in place of the factory Harmon Kardon sub ! The audiofrog was designed to use in an infinite baffle application! Meaning for use with no box ! After not being satisfied with this I added a Kicker amp ! Still not cutting it ! Long story short now.... My plan is to build a box in my spare tire compartment with hope of the great bass I'm getting out of my Sundown competition sub I have in My Buick LaCrosse ! Thank you Darren and Crutchfield for this great article!

  • Daniel Mcwrightman from Portland

    Posted on 8/25/2020

    I was just just about to recommend some mold release. I have used Part-all, or regular Trewax. Pva over the dried wax or cheap lacquer type hair spray is an alternative.

  • J Ro from Lexington Park

    Posted on 4/27/2020

    if i were doing this project, i would have ditched the spare entirely and tried to get enough volume for a 15 in there. so wow... your restraint is impressive!

  • Trevor from Johnson Nebraska

    Posted on 3/26/2020

    Is this box available for purchase? I have 2017 Alltrack

  • Michael from Walls

    Posted on 1/9/2020

    Left out important information. How many layers of fiberglass is needed?

  • Brian Thorpe from Indianapolis

    Posted on 12/4/2019

    I have done this is my Stratus a couple of years ago for a 10" kicker L7 sub with an Alpine amp. Most people think I have 2 12's. I also did this to make a box for my F150. And I am about to do the same for my new Sierra Crew Cab truck. I love using fiberglass.

  • Randy Toler from Lake Gaston, VA

    Posted on 11/15/2019

    I've often thought about doing just this in my vehicles over the years. I haven't yet as I've just gone the route of building custom wood/particle board boxes but that doesn't mean I won't in the future. I have quite a bit of carpenter skills so none of it looks too awfully challenging...just time consuming. One question I do have is how the wood front frame you placed in the vehicle as a wall actually attaches to the fiberglass. Do you fiberglass over the wood frame/wall and that in itself makes it structurally sound? If so, when you attach the wooden top of the box do you have issues with the fiberglass cracking once screws are used? Thanks!

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