Hello Billy Smart,

I install a piece of 1/4" Hardware Screen which measures 14 1/2" wide by 2"
tall
I install it in October and remove it in March.

BTW, in case you don't know it, mice can enlarge the entrance of a wooden
reducer by chewing on it.  However, THAT is not the reason I won't use them.
I want "wide
open" ventilation of the colony in the winter, because ventilation is so
beneficial to the natural living conditions of a bee.

George Imirie