Boom Mat 5"x7" Speaker Baffles
Reviews are mixed on what these do for sound quality, but I buy them for other reasons. I cut the back off of them, since they would be too deep and drag on the window anyway. I appreciate having the water protection over the speaker and that the outer part of the baffle provides a gasket between the speaker and door metal. Sure, you could cut your own foam or use foam tape for a gasket, but these are just cheap enough to add to a speaker order and not worry about it.
Pros: Speaker protection and mounting isolation from the sheet metal.
Cons: No idea if they do what they claim.