Check out "3-d Wizardry" (can't think of the author now). I
went to a workshop he did at one of the TAEA conventions. He
built his things out of thin carpet foam-type stuff. Like you
would get out of those kindergarten napping mats. I have a
pretty constant supply, since I ask the K teachers if I can have
them at the end of the year. I use the stuff for sponges for my
tables, but it's the same thing.