Why any particular screech owl becomes 'comfortable' with a box is still a bit of mystery. One of the reasons for pairing (circle hole and slot) is to see if this matters to owls. So far I would give a slight edge to the slot design, but very far from conclusive. Anyway….from Site 8.
After almost a full year an owl has finally tried out the ’round hole’. This was a one day trial period, however, because the same (i believe) owl has been using the slot box on a regular basis.