layers of papermachee. you only need to glue the first layer to get the mold of your arm (ball your fist) then when you take it off, just wrap and tape more newpaper around it till you get to the size you like. Do a few more layers of papermachee so its decently strong, and then cover it in plaster bandages.
one layer should work good, two for more strength, but the more you add the heavier it gets.
once the plaster dries, you can
1) cut out the newspaper , and insert a foam lining with a wooden dowl to hold onto the cast with.
2) cut out newpaper and insert foam and add buckles on the inside of it.
personally i would do 1, but thats just me