Straw, it's cheap & easy to get and it works. My indoor area is very basic. An elevated floor inside a bigger shed with straw. A hole in the side with a chicken ladder to the ground. No nest boxes, no perches. I put an igloo type doghouse in there as a temporary nest box until I could build some, but they didn't use it. They just went in the corner behind the igloo, fixed the straw to their liking & left the eggs. They go to the opposite corner which is closest to the door and line up along the wall to sleep. With no perch, they sit on their feet which keeps them warm. It worked so well I never saw any reason to change it. On extremely cold nights they will go into the igloo to sleep. On really hot nights they will roost outside on an old table or on the chicken ladder, unless we don't lock them in, then they may be out of the pen near the garden. One thing I noticed, without an elevated perch or box to roost on & no food or water inside the straw stays very clean. We occasionally add a few shakes & maybe once a year push it outside & put down fresh. About as simple & low maintenance as it gets.
