I mean, you could just buy a right-handed talon arm from the plastic daemon prince kit, off ebay?...had a quick look and found this one:
link ...I thought of maybe recommending an arm from the chaos spawn kit or something, but I'm not sure if any of them would be quite right, or big enough...and well, while I haven't actually seen the metal daemon prince in person next to a plastic one, I imagine they'd be about the same size, so the plastic daemon prince arm should work nicely, and match well style-wise