I used
Loctite Epoxy for plastic when I made my Spartan helmet (My Avatar). Gorilla didn't hold for me. I also filled seems with
epoxy putty which is sandable, and used a dremel to clean it up. Don't underestimate the powers of
caulk, either, for unsightly seams, LOL. I frickin' HATE hot glue and refuse to use it. If you incorporate foamies into it, use
Contact Cement and fill any seams with caulk. Good luck and can't wait to see your build thread!