DO NOT put anything down the cast to scratch! What you can do is take a blow dryer on a cool setting and let the cool air blow into the cast. That ill help relieve the itch. Also a very small amount of baby corn starch powder can be sprinkled in it.
Ok this one can only be answered by someone who knows from experience and that would definitely be me and the answer in an all plastics flyswatter it won't break and leave pieces behind and has no sharp edges that will cut and delicate enough not to rub your skin raw. Trust me I spent a year with eight cast on my leg