All Seasons Wild Bird Store (wildbirdstore.com) put together recommendations for nest boxes based on information from the following sources:

http://www.nabluebirdsociety.org/fact…​
https://bbrp.org/wp-content/uploads/2…​
https://bbrp.org/​

See also:
https://wildbirdstore.com/resources/a…​
https://wildbirdstore.com/2021/03/08/…​

This video discusses placement for bluebird nestboxes, selection of bluebird houses, monitoring bluebird nests, and resources for learning more about hosting bluebirds.

Special thanks to George Brown, the BBRP and Ray Marshall for photographs used in this video.