I was asked to make a bat box for a friends husband and this is what i came up with. I printed it in pla but would suggest petg for longer lasting outside use. On the print i have made two boards for the bat to use to climb up into the sleep slot. They are snug but do slide into the gap horizontally. I glued mine to keep them in place but they do stay there on their own.

The largest i went was 21 cm high but why not change the size if required.

Lets hope we get some visitors in them.