This!
Flying squirrels are very common but they are very nocturnal so people rarely see them. They are not very active in the cold (under 20 degrees) but they do move around some. I have a tray feeder attached to a tree that I toss some bird seed in at dark. The squirrels have the routine down and one can hear them sail in before I get 30' away. When full dark I flip on a flood light and watch them chase each other around and catch glimpses as the squirrels glide to and from the feeder.
Flying squirrels do not need much of a hole as they aren't much bigger than a chipmunk. I use bluebird houses as nest boxes for flying squirrels. They work well but one has to be diligent kicking out birds, red squirrels, and the occasional chipmunk.
