I would cut a piece of plywood the size of a super. Then cut a hole in it the size of the squirrel box. Take the bottom off the squirrel box and set it on the plywood and on the super. Put the super on a bottom board. Remove the squirrel box in the spring.
That sounds pretty good. I added/screwed a board to the bottom to close it up. This brings up another question. Will the bees build down? I do have a flow on right now but it will probably end on Sunday with a cold front that is expected. I will have to mount the hive box on the wall because the squirrel box is hanging on it, but that is not a problem.