Am I the only one with a copy of Super Simple Birdhouses you can make by Charles R. Self ISBN 0-8069-0858-0? - Published by Sterling Publishing in the U.S. & Canada?
With instructions for bluebird (or wren) birdhouse on page 53? (also suggests may be suitable for nuthatches, titmice & chickadees).
Only seen wrens nesting in my garden however, in a purpose built box/ with partial wall cavity behind (constructed when bricking up a door opening by our dining room window)which gives us all much pleasure seeing them use it every year.