I used a product I found on Amazon called FatMat.
http://www.flickr.com/photos/ajporsche/8709458344/in/set-72157632234070092

It's just like Dynamat as far as I know. Pretty easy to cut and install. Not really 're-positionable' so make sure you got it in the right place before you roll it down.

I guess it's not very cheap either:

http://www.amazon.com/FatMat-Thick-...391644660&sr=8-2&keywords=fatmat

Good luck!
~Dave