Have built several bat houses and they do indeed work at least for attracting bats. Not too sure if it cuts down on the mosquitoes though - kind of a tough experiment to do because you have to have a "before and after" readings!!
I got my plans online (several years ago so can't remember the site!).
It sounds as if you live in a rural area & before I built bat boxes, we discovered that we have a bat colony under the eaves of the porch and in an abandoned grain shed out back. We hung old grain sacks in the bin & there are hundreds of bats hanging around (literally!). Perhaps the same might hold true for your place. Sit out in a lawn chair when it's deep dusk but not completely dark & you'll see the bats flitting around. If you watch carefully you'll be able to see WHERE they're coming from.
I just did a quick Google search & here's a link for DIY bat houses.
https://www.pinterest.ca/habitatnetwork/diy-bat-houses .
Edited by RecycleDiva, 2019-05-04 12:28:36