I've seen other posts concerning rubber and cork stoppers I might also suggest the rubber like material used as a pinning medium for high school and college disection trays. This blue material is formed into sheets about 1/2 inch thick and can be cut with a sharpe blade. I have cut squares out of this material and placed them under components (with some sand loading on the top of the components: preamp and cd player)with results similar to my inner tube/mdf shelf/sandloaded chassis. I am a science teacher so the material is easy to come by. But, all science material catalogs carry it. I'm sure you could stop by your local high school and inquire about it.