built mine from a simple 6 x 2 and 4 x 2 frame, covered with uv resistant polythene, tacked to the framing and covered with a timber fillet to hold it in place, Still standing after 3 years on top of a very windy hill
I bought a load of electrical conduit from screw fix. far more than I actually needed (stupid minimum order quantities). It's pretty flexible, comes with lots of different shaped connectors.
You *could* use this to make a frame, but it would probably be quite flimsy at the size you're talking.
Failing that, there is no reason you couldn't use waste pipe of some kind, maybe 40mm, solvent weld together. If you want it more rigid, there is always the option of filling it all with crete, expanding foam or some other type of filler.
i think if you use anything like that it wont last too long. Polytunnels take a battering in the wind and the frame is under a fair bit of strain from the polythene as well. Unless you use metal or something similar i think its lifespan will be limited (says he who had one thin metal tubed one disintergrate in the wind)
might try 40mm waste have loads left from a kids goal making project (screwfix again).
If i push some 12mm steels in the ground then bend the pipes between these it could work.
Yes, must be careful of the winds tho.
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