I think I understand what you are asking, this is what I have done in my hives. One idea is to cut a strip of wood the same length as your top bar, make it 1/4" wide and as thick as your top bar. Put the strip in the hive right up against the inside wall, then insert your first top bar against it. What you are doing is creating a bee space between the wall of the hive and the drawn comb of the first top bar. Your top entrance will work fine then, the bees will have room to enter and access the combs.