Buy this one : www.waterproofcastcovers.org !This is far and away the best cover for a cast that I have found. I have suffered through a compound fracture and a crushed elbow. Both times I used a different cover and they were unbearable! I ended up cutting both of these in order to be able to use them at all. The opening was so tight it cut the circulation off and I had to cut the first one off. These were not cheap. However, the Castcoverultra is so comfortable and has a double closure at the top so your cast gets extra protection from getting wet. And the cost is comparable to all of the other covers for a product that is far superior.
I am a big fan of this and so is my mom, age 80, who broke her wrist. I was not about to subject her to the other cast protector and found this one on Amazon. I am very grateful I did.