I've fostered lots of broken legged IGs during wintertime, and Press 'N' Seal Glad Wrap is my lifesaver. While the casts and bandages I have to cover are not as big as you'll encounter, the Glad Wrap goes on quickly, actually clings, and will keep casts/bandages dry even in rain or snow. A sock pulled over the top will keep any exposed toes warm, too.
Good luck!