Bat Houses

I'm thinking of building some bat houses for my home and to sell.  Bats eat large amounts of insects each night.  It is a natural way of controlling the mosquito population and other bugs in your neighborhood.  Homes for bats are often in short supply.  Their populations are declining around the world, often because of disappearing habitat.

I haven't figured out what design to do for the bat house.  I'm looking for something that would  be simple, but yet visually appealing.  Of course, the design also has to be one that the bats would like and use.  Some possible designs are:

I like the third one with the gabled roof  the best at this time.  Let me know what you think?  What do you think of bat houses?  If you were interested in purchasing a bat house, which design would you be more interested in?