Plastizote/Plastazote - The latter is a trade name presumably for the former. I finally tracked down the manufacturer of this product which seems to be primarily used in the heatlhcard inducstry. Here's a definition from a medical dictionary of Plastazote:
Plastazote®
A cross-linked polyethylene block foam material used in the fabrication of podiatric and orthopedic prosthetic devices. Plastazote is a registered trademark of Zotefoams, Inc. (Zotefoams plc. in the U.K.), the sole manufacturer. The product is marketed/distributed by a number of companies, including JMS Plastics Supply (contacted for confirmation) and Ortho-Dynactive, Inc. Note: Some printed references as well as several web sites incorrectly list the product name as Plastizote. Evazote®, a crosslinked EVA copolymer foam, and Supazote®, a soft copolymer foam, are also trademarked products of Zotefoams, Inc.
If you look at the zotefoams website there's more information on the product. It's main advantage seems that it's very flexible, it's lightweight, it can be made into any shape and does not irritate the skin so makes it perfect for the healthcare industry.