The tunnel is up! It's not done yet, but we have already moved in ... much of what we seeded here in August was eaten by pests (squirrels got all the peas, slugs the rest) so we are transplanting things from other parts of the garden to winter over -- which is why the bok choi and kale at right seems a little wilted. Lettuce on the left is going great guns, though.
It seems to me the making of one of these is simplicity itself. Build two 3' beds and a 3' pathway between them, and when you're ready, get some ten-foot PVC pipes, snap them together in twos, mark off even distances along the length that you wish to go, punch holes in the ground with a hammer and steel pipe (we used a fence ram and steel fence post), stick the pipes in, bend them across the beds and stick them in the other side ditto, then harness everything together with 17 gauge wire. Cover with a single sheet of plastic (you can push it around overhead with a broom), hold down all the edges with bricks, tighten, and throw some dirt over the bricks. Height is seven feet, so unless you are a really tall Chinese basketball player, you won't have to stoop to work in here.
The ends are the hardest part. We've done ours with fence posts, 1X4 sill and lintel, and a piece of 3/8 plywood for the door, at each end. The plastic is cut down the middle, rolled up on laths, and the laths are wired to the fence posts.
All the veggies that will winter in here are also planted in exposed beds, for comparison. Last year, and the year before, we had enough of a freeze to make this worth while. The previous ten years we would not have needed it.
Considering all the hooraw among greenies right now about plastic bags, and rightly so, I feel a bit vulnerable to criticism for making such a big plastic bag. Arguments in favor of glass are convincing, but at current prices we did what we could do. It's intended to supply us with fresh greens through the winter, and maybe cantaloupes and sweet potatoes next summer. This is not the elegant way to do this, but it's a greenhouse for under $200. We'll see how it goes.