But all your hives have a top entrance, don't they Michael?
I'm trying to make robber screens that will fit every bottom board. And yes, I understand the mechanics of the things. I lost every hive I had in the backyard last year to robbers. We had a drought, unusual in Alabama, for 3 months, and I'm now surrounded by suburbs. I've had more problems here with SHB, DWV, and probably some alphabet soup I didn't recognize, than I have ever seen before. In contrast, I had a swarm with a very good, and well bred queen, move into an empty nuc box.
I came up with the idea of making screens like shoes. Different sizes.