mounting brackets do the job
Crutchfield Customer Review (What's this?)
they worked fine with my new Kenwood 5.25 speakers in my 2013 mazda 3 sport. I think Crutchfield needs to update its search criteria. the front doors are also 6x8 so i could have used a larger speaker but wasn't given the option
Cons:Crutchfield response on March 15, 2016 The customer was correct that the factory speakers in the 2013 Mazda 3 are 6"x8", but our OutfitMyCarT application didn't offer any aftermarket 6"x8" replacements due to the mounting depth and magnet diameter restrictions we measured in this vehicle. So, we showed the customer what did fit with the use of a bracket instead."