Bat Conservation International is the finest organization supporting bats out there. (I've been a member for years!) They are my go-to for any bat info, and their site has great info on bat houses including free plans to build them.
I would like to link directly to the site but am not allowed because I am a new member. I guess I'll be unrestricted from linking in a few days. So you can enter this URL listed here sans http:
www.batcon.org/resources/getting-involved/bat-houses
