a small top exit for the warm moist air is sufficient winter or summer but especially in the winter. Cross ventilation may cause more problems than it solves. I drill no holes in my top bar hives at all. The entrance is just the front bar back from the front 3/8" or so and that's the only ventilation.
So, do you not use a cover at all over your top bars? or one modified to keep that bit of space?
Sorry, no need to reply, I saw your answer to same question in another thread. if top bars are siting on frame, then cover is already raised enough to accommodate opening, if top bars are recessed, then prop the cover. makes sense.