Well see this is why pepakura weapons are somewhat discouraged, still many like me and my friend continue to make weapons out of pep, and its probably your best bet to not fiberglass it, while you can cut it in half and glass it that way, it is discouraged because it would be in 2 pieces and the 2 pieces may not hold together perfectly, or might not line up perfectly. Also you could try the expanding foam that was demonstrated in potshot's m90 shotgun thread as well as others, it worked well except for the fact that he filled it with foam before he resined to it deformed a bit. Just do the resin before the foam if you are going to try it this way, then it should work out fine, just poke some holes in the piece, (alot of holes) and then fill the piece with the foam. It will harden giving it some weight and making it a great deal stronger, but keep in mind it still is just foam, so if you actually try to snap it, chances are it will.
Just fill in the spots where the holes are with bondo when you get to that stage.