I scratch built a P-klaw for my biker boss out of plastic card. Didn't write up a proper tutorial, but here's a few pics to help you get shapes right:
Basically, you're making a large metal boxing glove, and adding some piston powered claw blades.
I also added a power pack on his back to provide the juice.
Conversely, you can always give him a buzz saw instead of the standard claw shape. With orks you really can't go wrong. Share pics when you get to working on it!