I had 12 traps out last year and got nothing. I had 16 out this year and got two. One of the two was in a trap that was right by my garage door and it had a round hole. It hung there since last year when it didn't get anything. The other swarm was in a medium with a 3 inch shim and a regular bottom board with the opening blocked to about 2 inches, so 2 inches by 3/8th inch. It did have a flaw in the shim that allowed about 3 bee size holes where the shim had seperated a bit. When the swarm arrived they used all openings to get inside.
I can't say that I believe a round hole works better but I don't really know. I say what matters most is how many bees swarm in your area and how many other choices they have for homes. I got all excited getting two swarms in two places 8 miles apart but when I checked the other traps, nothing was happening. The one that moved into the box by my garage had another box exactly like it including how high and entrance direction. I hung them on two trees expecting to move them to good places but never got around to moving them
I do know the bees were checking out my hives I have set up for when I get bees. You could definatly tell which one they were liking best though.