Danno, what are robber screens, please?
a robber screen gives the colony full ventilation and a large entrance but a very small entrance to guard. I know that doesn't make sense so let me try to explain how I make mine.
2 pc- 1/2" X 1" X 4" pc of pine
1 pc- 1/2" X 1/4" X about 14" pine
fasten the 14 " pc to the FACE
of the 4" pcs to form a u shape
now staple 1/8 mesh wire over the face of the U
turn the U upside down and screw it over the entrance leaving a small opening of about 3/4 on one side. This 3/4 opening now become the main entrance but colony bee's very quickly learn that the whole top 14" has a opening and they start using it. any robbers will land on the bottom board
(where the hive smell is) but will only be able to use the small 3/4 opening on the end. The colony can usually defend that small of a opening