I have fantasized about a greenhouse for a long time. And last year I did manage to drag up most of the materials I need.
I keep my eyes peeled for inspiration. Here's one, thirty miles south of my place and just a couple of feet above sea level, that obviously hadn't been used in a long while. It's interesting because it was, though it's hard to tell now, quite carefully made but, somewhat more obviously, at low cost. The roof and some of the walls were corrugated fiberglass of some sort. Greenhouse roofs here don't really need to be transparent, as the sun is up all day in the summer and very low on the horizon in spring and autumn.