There are a few different ways to create texture on the inside of the bat house. Rough texture is necessary, it makes it easy for bats to grab on and nest comfortably in their dwelling. One method is to staple plastic mesh on the inside of the panels. The alternative is to cut small 1/4” horizontal grooves in the wood, making a ladder on which the bats can climb and cling.
A palm router with a 1/4” straight bit is perfect for creating horizontal grooves. Use a piece of clamped wood as a straight edge and set the router depth to plunge 1/4” deep (the grooves will be 1/4” x 1/4”). Continue routing these lines from the top of each panel, towards the bottom.
Adjust the router depth to 3/4” and use the tool to create a ventilation strip 6” from the bottom on the front panel. You may find it easiest to mark the location of the ventilation strip and then re-route the opening once the house is assembled so the straight bit can plunge through easily.