I built robber screens from the book Queen Rearing and Bee Breeding for all my hives and they really work well. It narrowed all my main entrances down to 3/4 of a inch with total ventilation. The bee's can exit across the entier top but all bee's trying to enter land on the bottom board and single file in the 3/4 main enterance
They work very well don't they.
As MB stated these work well and are something to look into. It did tick them off for a day until they figure out how to get in and out