This is a link to a Google 3D SketchUp drawing for a bat box designed to slip over a 2.5 inch O.D. pole. You will need the SketchUp software to download this drawing and its freely available online.
Build a wooden box sleeve to slide over a 4×4 post that has been notched out using the plans at Bats North West.
At the link you will find basic information for building a bat house from DCCL.
This bat box house plan has been successful in attracting nursery colonies of little brown and big brown bats. Bat roosting requirements are strict, necessitating adherence to construction details. Free plans at Wisconsin DNR.