i would suggest making a few small holes and filling it with something. you could use any number of things....resin or "mud" would be really heavy but sturdy because it would be solid plastic. or you could use low expanding foam. resin the outside a bit to make it strong and CAREFULLY fill it with foam. i stress carefully because i ruined a perfectly good M6D because i was being impatient. the foam can easily warp your piece when it expands.
Wow, great job. I really respect you guys who do Pep weapons, they look nigh impossible hah. Today I saw an AK-47 in the shop of some store, I can't imagine how long that would have taken. Keep up the awesome work!
Some people have done this and I might as well : just fill the pep with resin a bit after resining it a couple times on the outside. Then, slush the resin around to get an even coat. That's what I plan on doing with the AR i'm making.