I have a decent sized notch in my covers that I push the telescoping cover back against in wintertime. I move it out in the spring -- it does not seal up tight, so I get some ventilation but not a howling blast of cold air through the hive.
I always have plenty of bees up on the top of the inner cover drying honey in the summer, I feel they do better with the extra space. I do have a bee escape hole in the center of the cover.
I firmly believe that a double cover provides better conditions in the winter, only a solid slab of wood or a quilt box is better. Wood absorbs quite a bit of moisture before it shows condensation on the surface, and even if wet is unlikely to drip. I have a 1/2" thick inner cover, a space around 3/4", and then another 1/2" of plywood with about 1/2" of newspaper on top under an aluminum cover. Quite a bit of insulation compared to a migratory cover, and no water at all gets in.
Peter