Material for fabrication (not specification) templates

What is a good (i.e. readily available, cheap, durable, impervious to water, easy to fabricate, adequate thickness, etc.) Material (http://g654.org) to use in the shop for fabrication templates?

To be clear what I'n talking about...
Whenever we need to fabricate a long curved run, we make a fabrication template out of solid surface Material (http://g654.org) that we've salvaged from a previous tearout. We're not solid surface guys so getting our hand (http://maplered.com) s on this stuff is iffy at best.