Foam - Mate! Cuts FAST and has lost of cheap replacements. f your first time use some cheap Blue Foam (Home Store - of choice) glued up and cut to make sure its how/what you want. Then you could invest in REN Tooling board - or do the seal and layup on the blue for a real test. BTW the foam makes a BIG MESS so MASK-UP!
You can "protect the foam" with a variety of means; tape, acrylic house paint, paper and white glue, etc, etc.
Be sure and test if your foam is compatible with epoxy -- or polyester resin -- find one that works and press on. You didn't say what the product is to be used for - so either epoxy or polyester "may" do the job.