I’ve never had a cast, but I always hear how much they make the skin underneath itch. What if they were made with plastic tubing embedded in them, next to the skin? That way, cold or hot water could be run through it, which might relieve the itch.
Or, the plastic tubing could be withdrawn after the cast dries, leaving an “access channel” that spirals around the limb. Then, when the itching starts, you could snake in a long pipe cleaner or something like that to scratch it. As long as the channel isn’t very big, the cast itself should still remain strong and rigid.
That seems to make sense to me. Here are two enjoyable cast-related photos for you: