I used a product called Bituthane(might not be the correct spelling). It's used as a water/ice barrier on roofing and concrete applications. Picked up a 12 inch wide roll believe 50 ft.long for $35 at a roofing supply store. It's super sticky like tar on the application side and smooth on the other. It's a great insulator and will reseal itself when using hardware through it. It does peel a little at the outside corners when it gets hot like when applied to the firewall, but I haven't noticed a smell and put this in 3 or 4 years ago. My truck is a daily driver so I would noticed.
