Scott Sanderson produces a very accurate pair of solid cast resin wrists.  With Scott's permission I have created lightweight fiberglass wrists based upon his design.  These are very strong but much lighter than resin. (A single fiberglass wrist only weighs about 12 ounces!)  I'm trying to keep the weight down as I hope to animate the Robot's arms eventually and so the lighter the better!

Here's some shots of the wrist plug and the mold I made from it.  The mold is three piece.  The center hole gets cut out and Scott's rotating piece fits the center, just like in his resin design.  The mold was constructed using the same methods as when I made the torso mold.
 

Here's some shots of the actual wrists.
One has been primed, the other has been painted the "Gun Metal" color.

Before and after cutting and trimming: 

Here's how Scott Sanderson's rotating center and claw bracket fit:   

Each wrist weighs ~ 12 oz (Not including the rotating center section, claw bracket or claws.)