When I built a similar but smaller “hoop house”, I got the longest rebar I could find, then got PVC tubes that were about a foot shorter than the rebar. For each hoop, I jammed one end of the rebar in the ground, then threaded the rest through a PVC tube, then jammed the other end of the rebar into the ground. I used another piece of rebar as the top ridge line, and used zip ties to attach the hoops to the ridge line. It worked pretty well — the biggest problem was remembering to leave the door flap open for ventilation on sunny days!