About 5 years ago I bought some stuff on ebay for sound/heat insulation. It is some sort of foam material covered with aluminum foil on both sides. The foam is similar to white foam packaging material; not bubble wrap, not styrofoam, somewhat rigid, not easily compressible between your fingers. The stuff is 1/4' thick & came in a roll about 5 ft wide & cuts easily with a pair of scissors. I put the stuff down on the floor against the sheet metal, on the firewall, and on the roof above the headliner hoops. Didn't even have to glue it in; curving it around the sides along with the hoops seems to hold it in position just fine.

I don't know if it is as good as other well known products, but as I recall it was substantially cheaper. That's me - always trying to save a nickle here & a dime there. It's like I tell my buddys about owning a Norton; parts are cheap but the problem is you need to replace so many!