Why not just use body filler? If the hole is all the way thru to the firewall, you should probably use some tape or penetrating oil on the firewall metal to keep the filler from sticking, but then just mix some filler and apply to the area. The filler will make a decent insulator. Use some sandpaper and a stiff brush to texture the finish of the filler, and then paint to blend it to the finish of the insulator.