I built one that fit right over his dresser so it saved a lot of floor space.
Pretty simple to do.
Built 2 end frames out of 1x4 pine with, tongue and grove wall paneling sandwiched in to close off the ends, box for the mattress out of 1x8 pine, attach the mattress box to the end frames, sheet of thin plywood on the wall side for rigidity and a strip of 1x4 cedar on the front with brass screws for a nice contrast to the pine.
I built this about 12 years ago and it's still holding up well with my 5'10", 200 lb teenager