i build a 4 hole nest from about 2/3 sheet of 1/2" plywood. the boxes are 12"x12". the front side is 14" tall and slopes up to 18" to keep them off the top. there is a 1x4 across the front to keep things in place. I found a similar pic here in byc.
construction only took about an hour.
the box sits about 2' off the floor. I didnt put a bar in front but am considering it to ease the efforts to access although so far, ive had no problems, the girls are getting in there.
i dont think it matters how high up they are, if the chicks want to roost in them, they will. I have one that does and a friend of mine has 5 or 6 that do. Id like to break them of it, but not sure how to do so since the house is not in my back yard.