Well, DUH !!! I just realized I've been walking right past a cheap suitable solution for the last 3 years. Along one back corridor at work there are several large recycling receptacles for various materials, sorted by type. One container has all sorts of packing materials like tough industrial bubble wrap, sheets of closed cell polyfoam of various firmnesses, foam blocks, etc.
I do like the material Sinecure mentions... but the stuff I can scrounge is FREE!
My first thoughts were to buy a couple sheets (if I could find) of the neoprene that diver's wet suits are made from with fabric on one or both sides.