The absolute best I've found is "black hole" acoustic foam.
It's several layers bonded together
- adhesive (peel and stick)
- dynamat-type bottom layer
- thin lightweight foam that simply decouples the limp wall
- "limp wall" layer of vinyl (or some soft plastic membrane that seems to be vinyl)
- thicker absorption layer
There's only three problems with it
- it's thick (so you need to put it into a large case
- it's kind'a expensive
- the company seems to be out of business (or at least not selling to the retail market anymore - this stuff is normally used to damp standing waves in speaker cabinets - and they may still be selling to speaker mfgrs...)
I'd suggest you do a web search for "limp wall" and acoustic. If you're really interested, try F. Alton Everest's
Master Handbook of Acoustics
Bob