Em,
The reason I suggested you carry your board & vinyl to an upholstery shop is that they keep a gun loaded with Weldwood's Contact Adhesive for Landau Tops or something equivalent from 3M or the like, which is the good stuff:

adhesive1.jpg

This stuff is made to withstand the heat, and is NOT the same Contact Cement you buy at box stores. It's made to be sprayed with an old siphon gun to get a thin, even coat of glue on the two surfaces. The good thing is that your package tray will have the edges rolled under, and you can staple them which should keep it all tight, along with the grilles on top.

I like the idea of the "tupperware" containers on the back side to give the speakers something to push against, but I'd go by WallyWorld and splurge for a couple of new ones vs pulling something out of the cabinet which might have you wondering why your car smells like leftovers when it sits out in that hot Texas sunshine. Just my thoughts....