A while back I wrote to the guys at Clam Outdoors and told them to look into developing something like this for cargo trailers (no affiliation):
http://stores.clamoutdoors.com/flip-over?_ga=2.260121871.348724628.1526586810-577660604.1526586810Basically, the drop down capability that could attach to a cargo trailer rear entrance or go up and over the rear of the trailer to secure it. Get them made in 6', 7', and 8.5' widths. Add doors/windows etc to make it more usable since we're not ice fishing.
Using the following picture, imagine the trailer being where the boat is.