Mark H. asked:

"What would be the best way to provide this ventilation between brood boxes?
A drilled hole? Some other way?"

I drill a semicircular hole front and center on the bottom half of the inner
cover.  The way to do this is to clamp together two inner covers and drill
the hole so each cover gets half of the hole!  Bees (even package bees
accustomed to the availability of a bottom entrance only) immediately prefer
this simple top entrance and use it almost exclusively within a week after
being given a choice of top or bottom entry.

This top entrance also provides all the winter ventilation needed.  A piece
of blue stryrofoam-type insulation between the inner cover and the
telescoping cover completes the job in winter.  Ask me for a digital picture
of this arrangement.

Jeff Hills
Hills' Pure Gold
Dorset, Vermont