I'm here now!! ....
not that what I do will work for you, but it may.
Its worth putting up something now cause the young will be searching for nest sites before they migrate.
The pic was taken today, one of several of the same, though a new shape this year. I will go back to the flycatcher letterbox next year however as they are a bit better.
Nothing has to be done to the boxes, they will build up the mud themselves and the nest lining.
Basically the better design is the same depth hole as in the pic but cut all across instead of a quadrant. they will build up their own entrance out of mud.
however, the young are using the box in the picture too.
Its important to leave the space above the roof forthem to build their own. but this way their nests dont collapse in bad weather and the owls can't get them.
I will have another look later for pics of the previous years boxes .