Brownmed SEAL-TIGHT Original Cast and Bandage Protector, Pediatric Large Arm
Don't fumble with plastic bags and tape to make a cast waterproof - use SEAL-TIGHT coverings instead. The key is the SEAL-TIGHT watertight seal, a flex-seal rubber diaphragm that slips over the cast and fits your leg or arm squeegee-tight.  The diaphragm creates a uniform seal around the limb and permits drainage of water away from the cast protector.  The vinyl arm or leg bag is durable; it is abrasion resistant and minimizes slippage while showering.  The ring system, which connects the seal and bag, is welded ultrasonically, and supported by caulking and rivets.  With 14 sizes to choose from, you'll find the exact size you need.  SEAL-TIGHT will withstand daily use during the life of a normal cast.