I'm using a 5x7" speaker. The mounting is stupidly complex as it incorporates the two screws that hold the trim to the door. Therefore there is not simply a hole for the speaker but instead a series of steps and large gaps.
I've just come up with a solution of sorts, involving foil tape and knead-it epoxy (the dark grey stuff), then some foam tape to try and get a level 'ring' for the speaker to sit on. The two trim-fastening screws are still a problem as obviously they must remain accessible.
It has made a huge improvement to the sound quality - just try holding a speaker in mid-air while it's going and you'll know what I mean. The speaker sounds much less like a tweeter once it's sealed (or at least, semi-sealed) into the door.
The OEM approach was to seal the grille onto the front of the speaker, effectively making the grille a baffle. Very odd and not really effective!
It is amazing what a balls-up can be made of simply having a speaker stuck in a hole - gone are the simple days of a hole in the metal, cut a hole in one thickness of hardboard and vinyl, make sure two of the self-tappers go into the metal, and Bob's your Uncle.
Anyway I took a few pictures but can't upload from my iPhone; will try from my PC tomorrow.