One of the best ways to reinforce firewalls is to glue (with epoxy typically) one or two layers of fiberglass cloth over the area you want to reinforce. You can buy 6 ounce fiberglass cloth at Wal-mart in the automotive section. Soak the glass cloth (on a piece of waxed paper) with epoxy, squeegee off the excess glue, then press it on over the area.