ummm personally i would make a bulk papermachee of your hand. so youd cover your hand with a few layers of newspaper and glue. let it dry, and then peel / cut it off (id suggest to ball your fist for that so you don't have to worry about fingers) then once that's off just wrap, dont even need to glue newpaper on. once you get it to the thickness you want, add one more layer of glue papermachee to seal / harden it a little.
to get the cast look / texture, use plaster bandages.
things are real cheap and probably only need one roll (CAREFUL!! these get really, really heavy when you start adding lots of layers.)
P.S. try and drain as much water from the bandages as you can, it just helps dry faster.
after that its up to you if you want to
1. remove the newspaper and line it with foam and a wooden dowel as a handle so its a slip on / off
2. keep the news paper, drill a hole to insert the dowel and just seal it up.
3. what ever else you can think of~
oh also, if you want to you could cut it in half and add buckles. but i personally think the slip on / off idea works the best
good luck and cosplay on~!