Try a rope thru an overhead pulley. Use the good arm to gently pull up the other to extend range of motion..
I googled a stretch that involves walking your fingers up the wall. Pretty effective, but range of motion is pretty pathetic.
I saw someone at PT who had done the walking the fingers up the wall improve more rapidly by doing it in the hot shower. Hope you get better quickly!
(Everyone else's comments were much more helpful …)