How about using some spray-on undercoating/dampening? I used to own a Jeep Wrangler, and one of the options is to get a hide-a-trunk. I put a bass tube in it and it rattled like crazy. So I took out the "trunk" and lined it with the coating. Rattling toned down a bit. Also, I had a hardtop and didnt notice any funky smell from the coating.
Just a thought, cost only a couple of bucks.