[crosseyedsheep]

Here's a link to a a homemade poly tunnell, it looks pretty good, I think it would help a lot if you could get a proper steel support for each end, I'm not sure if you would save a lot as you're only substituting the frame with an alternative, you'd still have to buy the same cover, it would be interesting to do a price comparison:

Homemade polytunnel
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Xeyedsheep