another alternative
I've got 2 12" IDMAX's fiberglassed into the left rear wall of the car and I used to get the same gas cover rattles you're talking about. I contemplated Dynamatting, but decided to try something simpler first.
I went to Target and picked up a couple of rubber bumpers -- they look like little clear rubber circles, about half a centimeter high and a centimeter in diameter. You normally use these things on cabinet doors to stop them from slamming. Anyway, I found that the rattle is mostly caused by the locking mechanism inside the gas cover. So I place 2 of these rubber bumpers inside the gas cover to provide a little more resistance. Now, the door closes normally, but you just have to push on it a little harder for the lock to click. With this added resistance, the gas cover doesn't make anymore noise, even with Lil Jon turned all the way up.
Now if only there was a way to silence the rear windshield wiper ...