Way back in the day, 1980ish, I got a Tamiya FOX RC as a kid. My dad did most of the building. It came with a rubber balloon to put the controller into! I wonder if you could make the RC Shermans waterproof like they did during the war, just by sealing all the seams in the hull. The crews had to do this to their tanks in many cases. I don't think they did any factory waterproofing until fairly late in the war.