I haven't looked for any more websites showing PVC "hoop" plans, but the one I linked to would be described as "pretty fancy" compared to the basic "Okie" version mentioned. Just pound pieces of rebar in the ground and slip a stock 20' piece of Schedule 20 plastic pipe over the rebar. Line up three "hoops" and tie the tops together with baling wire and guy wire them to stakes in the ground. I suppose the sheet plastic should be held on with small metal hoops so that the plastic can be raised in hottest part the day. Details needed.