Much simpler just to use heavy duty rubber (or similar) and fill it with a non-Newtonian fluid. Think cornstarch and water, or oobleck.<p><a href="http://www.google.com/search?q=oobleck" rel="nofollow">http://www.google.com/search?q=oobleck</a><p><a href="http://www.youtube.com/watch?v=yHlAcASsf6U" rel="nofollow">http://www.youtube.com/watch?v=yHlAcASsf6U</a><p><a href="http://www.youtube.com/watch?v=fYj6Ju8IcW0" rel="nofollow">http://www.youtube.com/watch?v=fYj6Ju8IcW0</a>