Here's what we used, we got ours through the Barn Owl Project run by the Essex Wildlife Trust. It then gets monitored by them and then, as happened this year when we had 4 chicks born, they all get ringed and registered.
We had a roost in a barn adjacent to the one we developed, so knowing we'd disturb them we put up a couple of boxes in an adjacent hedge row. The Owls used one of the boxes as a breeding site in the second year it was up.