There are many possible designs for bat houses. To increase the chances that your bat house will be occupied, consult the following page "Setting up a bat house" in order to position the house in the best place possible.
You can also find one-room bat house plans at the following links:
https://www.batcon.org/wp-content/uploads/2020/09/4-Chamber-Nursery-House-Plans.pdf
and
https://batcon.org/wp-content/uploads/2020/04/RocketBoxPlans.pdf