What a timely post! I just made one myself about a month ago to hopefully get rid of some of my mosquitos. So far, no bats though. I have seen bats in my yard before, so I remain hopeful. I followed plans from the Missouri Department of Conservation. It was a pretty simple build, although I did modify the plans a bit. Here's the
plans. Apparently a bat roost this size will hold somewhere around 80 bats.
The bat symbol was a nice touch (just in case the bats got confused as to who it is for). My worry is that wasps will move in before the bats.
In my research, I learned a few things, some of which has already been mentioned.
1. It is very important to have the bat roost south facing. If it doesn't get enough sun, the bats won't be warm enough and won't stick around.
2. It is good if you have a large body of water within 1/2 mile of your house to serve as feeding grounds.
3. Apparently it could take up to a year or two for bats to find your roost and move in. So, I wouldn't get discouraged by not having any bats quite yet.
4. Trees are not ideal to put the bat house. They may not get enough sun and they leave the bats exposed to predators. They also should be at least 12 feet above the ground. I hung mine on my chimney.
That's all I can think of for now. Let me know if you have any questions. Good luck!