From White Pine Barnboard
Two bathouses can be built in 2 - 3 hours or less
Bats are indeed making a comeback in Wortley Village
Some builders add a piece of window screen to the inside to help bats climb into the cavity, but I cut grooves at 3/4 inch intervals (with table saw) on the inside before assembling all the pieces. I suspect wee bats can clamber into the cavity without this assistance but it does provide another talking point when someone shows interest.
Please link to Bats Making a Comeback?