I made a copy of the small Prime rifle that came with the Gear of War 2 set and used some Fixit Sculpt to give it a regular sized post to fit into WFC Prime's hand (or any standard fist hole). Fixit Sculpt is sweet stuff, might I add. People rave about it and for good reason - it's great. I started with a straight cast of the original, used Fixit to fill in some bubbles and built/sanded the post.
I think it came out pretty decent. I really wanted to use Smooth-on's Task 9 for the resin, as 1) it's clear, which I was hoping to tint a clear red to see if that would look cool under the black paint and 2) it's strong as hell. Unfortunately, I've been fighting with bubbles and curing issues with it so I had to use the last of my Smoothcast 300 instead.
So, I've casted it and primed it with Fusion Camo flat black and hope to go over it to smooth out anything I missed when it was white. Then finish it off with some highlights/dry brushing. I don't have any other Primes at my house, like Classic Voyager or the other deluxe spinning-punch-action Prime, but I assume it should fit those too.