I had a go at the power fist with, I'll admit, not the best of success.
What I found particularly difficult was :
a) The original image is so difficult to interpret
b) There's no real life equivalent to compare to (like with guns etc)
Anyway, here's my first attempt next to original :
Given its in-game size, I've kept the polys as low as humanly possible - should be about 62 triangles there.
It is of course, blocky, and likely inaccurate in places, but in game might not be so bad, if you look at this approximate diagram :
If it's of any use, the model is uploaded here, as Wings, 3ds, obj & texture :
http://www.megaupload.com/?d=81GA8ZUOI tested it with Karpov's hand model - it will fit over it, though it'll need rotating and resizing, and possibly hollowing out, depending on how the models are meant to fit together.