I used a product called eDead from Elemental Designs . Worked great, and was cheaper than the dynamat/B-quiet/fatmat. I heated it up slightly with a heat gun, then used the blunt end of a screwdriver to push it into all the corners and contours. (
www.edesignaudio.com) I decided on this b/c it wasn't tar-stuff (no smell), and was dual-duty (heat and sound).