Any ideas how to keep leg casts dry in the winter?

My 5 Y/O son is having surgery for club feet, and he will be in leg casts again for about 6 weeks. This will be from Dec 20, until around the end of January. Like most other 5 Y/O boys, he likes to go sledding and play in the snow, and just be outside. Anybody have anything that will work for him to keep his casts protected?