tips for building bat houses - "chambers at least 14" wide and 24" tall (bigger is better), and a wide landing area with a very rough surface beneath the entrance. Do not use pressure treated lumber. These furry little creatures prefer temperatures between 85°F and 100°F."
Install a bat house. Plans via pdf free dowload included. (The UN has declared 2011-2012 the international year of the bat and encourages people to get involved. Their numbers have dwindled from white nose blight. Their importance to the any ecology is often unknown. In short, they'll clear your garden of bugs in one night if they're around. :D)