couldn't you make the spacers out of wood? I got a jigsaw for 20 bucks at walmart and it has helped me do a lot of things like make butch plates for the 6x9 boxes that I also got at walmart another good thing to do is get the sticky foam sheets in the craft dept there for like 50 cents and slapem on either side of the spacers that you make so you get a good seal the foamies compress when you screw it all together and keep the wood from vibrating and stuff I think that is what you are trying to do right? get the speaker out off the door more so you have enough mounting depth? I am planning on stuffing a set of infinity 4x6 plates, just like the ones i have in the rear deck, into the 5 inch openings in my doors so let me know what you come up with I think I looked it up in crutchfield though and the mounting depth will not be a problem. oh yea walmart has the biggest database in the world next to the fedz...scary huh?
Refs: Diyma.com, Caco.com