I may or may not be able to help make your decision here...
I just posted a pic on another post of a Boba helmet I'm working on for my son, using this very technique. It was originally all posterboard with fiberglass and resin. I ended up using sheet styrene for the outer layer on the helmet. I think it'll hold up quite well.
That said, I'd planned to do the same with knee armor and the cod piece and they didn't hold up. What happens is it'll flex enough to rip the posterboard apart. From here on out anything I make will be styrene and fiberglass/resin. The cost isn't much more and it's less temperamental. The styrene I use is so thin it cuts easily with scissors.
