Since the original plastic astronaut helmet is made out of poyethylene--a rather slippery synthetic--I figured I could make the fiberglass directly over it. I took a piece of 5" wide nylon stockinette and it just stretched over the plastic helmet. Then I saturated it with polyester resin and after curing I had a thin strong form that matched the inside of the Explorer's helmet.
I cut the fiberglass shell in half, removed it from the form, and trimmed it down to fit my leather helmet. Above you can see it being contact cemented into the helmet.