I used the knock-off dynamat type stuff (sticky backed thick foil), was $25 a roll at a local parts house. Covered the firewall with it before I put the AC in. The big sections I also used the foil backed bubble wrap, glued it to the other stuff with 90 weight 3M spray glue.

Seems to have done the trick for me, and the firewall section cost less that $30 that way.