I did a similar thing to Audio Express in my apollo (same as gen 3 camry) to fit a pair of slimline fusions.
Took the oem plastic thing out, made an mdf plate (only 6mm i think) and designed it to fit the factory position/screws (counter sunk the original screws so that it bolts into the original screw holes as well).
I would recommend this only as a temporary/quick fix solution as at higher volumes I think the woofer starts to hit the back of the door trim, and the front windows dont go down completely anymore but im too lazy to fix it

Audio Express's cutting metal is most probably a better way of doing it, but at the time i just wanted the speakers in the car

(that and they were fairly cheap to begin with so i really didnt care

)