DaveNH,
The Missouri Department of Conservation <A target="_blank" HREF=http://conservation.state.mo.us/>http://conservation.state.mo.us/</A> had a design fabricated from 2x2's, 1/2" plywood, and old seatbelts. I made a couple and they worked great. Thew were just a single platform, though, and not very comfortable. A quick check of the web site didn't find it online, but maybe you could call and they'd send you the plans. I don't know what I did with my copy.