The trouble with many commercially available bat houses is that they are just too small to house a colony of bats. And it turns out bats are pretty finicky about temperature, too, so you'll need to select the color for your bathouse depending on where you live because it will affect the temperature inside (dark colors absorb significantly more heat from the sun.)
The step-by-step instructions we found over at the National Wildlife Foundation explain how to make one that's big enough. Sure, it calls for an entire sheet of plywood, but most lumberyards will cut one down for you. (Tip: use a sheet of grooved plywood -- sold for use as house siding -- and skip the step of cutting grooves.) Then all you'll need are common hand tools, some screws, and a tube of caulk.
Another good resource: Bat Conservation International's Guide to Bat Houses.
image by weirdvis via sxc.hu