I use this rubberized foam. Its used in hobby gear for dampening vibrations. Its pretty much memory foam, just rebounds faster.
http://www.amazon.com/Du-Bro-514-Protective-Foam-Rubber/dp/B0006NATDY
http://www.amazon.com/Du-Bro-514-Protective-Foam-Rubber/dp/B0006NATDY
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