What is the recipe to make plasti-goop for the old Creepy Crawelers toy?

Mattel, and later ToyMax and Jakks toys manufactured a toy called Creepy Crawlers in which you poured liquid plastic called plasti-goop into aluminum molds of bugs and other creatures and baked them in a small oven to make flexible, bendable toy bug. The refills are hugely expensive (even on ebay) so I am looking for a way to duplicate the recipe for plasti-goop. I am not looking for alternatives like clay, elmer’s glue etc.

Does anybody know what plasti-goop is made from?