Although he doesn't specifically say what it's made of, I'm thinking after poking around the site that it is plastic, not fiberglass (I know fiberglass is technically a type of plastic, but you know what I mean).
Here's why I say that:
1) He mentions in a few places that it is "plastic" "foam filled plastic" etc.
2) In another section, he talks about his design goals, and how they could have been met by utilizing "either plastic or fiberglass."
This leads me to believe that when he says "plastic" he is not talking about fiberglass.
I think it's a super-cool idea - would love to see one!