I hear that corrugated iron sheeting is great, heats up a treat and even comes with drainage/fixing holes ! If you can get a few bits of angle iron then you don't even need any wood for uprights. Don't know about the iron content of your spinach though.... And maybe you could overdose on the zinc ?
I was lucky enough to notice a double glazing firm who were replacing the soffits and fascia boards on a house. I not only got all the old guttering for free, but also the old wooden boards they were replacing (nicely painted, thank you,) and the offcuts of uPVC boards they had, which would otherwise have gone to landfill.
I have my doubts about the plasticisers in such things, Bisphenol A and the likes are getting a lot of flak nowadays and those world leaders in toxicology the Swedes (human cultivar ) have banned a lot of them, but with their high insulation value they do have a good reputation for growing things, that's for sure. I am certainly keen to see my local glazers again and see if they can supply me with more offcuts for coldframes etc. Given the soaring landfill tax, I think they are likely to be amenable !