Yeah, your best option would be to use regular fiberglass resin. Of course, if ventilation is a problem then you wouldnt be able to use resin. There are a few options though, Ive seen a few tutorials around here on using wood glue to harden peps, as well as a hot glue method thats on the halo costuming wiki. And im sure theres a few other methods as well. Good luck with whatever you choose!