I'm using an acoustic/thermal insulation product called ZeroClearance...it has a heavy foil and silica/polyester with a super sticky adhesive. You cut it to fit, peel the backing, and stick it on. It'l help keep heat out (GT40s can get toasty inside) and I hope it'll keep the interior reasonably quiet and prevent drumming of the larger panels. I'm using it on most all of the panels that face the interior. You can find it at
www.coolandquiet.com. I'll probably add about 15 pounds to the weight of the car with this stuff.