I think I saw Norm Abrahm build one once. Seems like they were pie shaped boxes. The box design uses a lot of wood though. The height of each box was the heighth of the riser(6" to 8"). On the outside, the depth of the box was about 3 times the tread depth (10" to 12+"). As each piece of the pie is added on, it would stick out past the lower one about one tread depth on the outside and still had twice as much to nail/screw to the lower one and to help support it.
I could draw one and experiment to make one work but I keep thinking about all the wood it would take. Get a welder to weld some brackets in a spiral onto a piece of pipe (4" to 6") to make your staircase then bolt your treads to the brackets. That sounds cheaper and easier to me. Good luck.