Barn Owl nestboxing

BEFORE
Quot homines, tot sententiae definitely applies to the design of nestboxes for certain species. The best accommodation for the Common Swift (Apus apus) for example, is a subject as contentious as Princess Diana's last day on earth. Do Swifts want big or small? with or without a nest cup? an oval or a square entrance hole? the hole on the side or underneath? a light or a dark interior? gas-fired or oil-fired central heating? - seriously, there are as many opinions as there are Swift enthusiasts.
You would think that there would be unanimity of opinion about the preferences of Barn Owls (Tyto alba), but even in the case of this species, there are many nestbox variations. Take the bog standard square box type - think of a tea-chest with a front on it. What size should the entrance hole in the front be? Not too big - don't want draughts, but not too small either. Where should the hole be? At the top, at the bottom, in the middle? Should there be a "tray" in front, and if so, should it have a raised rim?
OK, for those of you who are about to put up a Barn Owl box in your stable, put the hole at the top, so that the young cannot get out until they have at least some flight feathering. Otherwise, if they fall out, they will not so much flutter down as plummet. Make sure there is a tray so that when the young do emerge, they have an exercise platform, but whether it needs a parapet I cannot tell you. Oh yes, and there is no need to write ENTRANCE over the access hole.
I have just completed my nth Barn Owl nestbox, so I do know whereof I speak. What I do not know whereof is why, no matter how carefully I measure up before cutting the wood, nothing ever fits. If I could grow instant 400-year-old oaks with nest cavities, I promise you I would never make another box.
AFTER