I have already checked out the dental impression material. It's call vinyl polysiloxane. It sets up too quickly and for my purpose is extremely expensive.
I looked at some ski boot ads on-line. Some require heating their bootie to 250 degrees in an oven before putting your foot (w/sock on)...
Here is the problem: The bars of a saddle, which rest on a horse's back, rarely completely conform to the exact shape of the horse's back. I want to make a rubbery or heavy dense foam shim which will fill in the voids between the saddle bars and the horse's back so there is a perfect fit...