This plan was posted on the Homesteading Today forum:
Here is what you need to do.
Lay out the two sides of the structure on the ground using marking paint or chalk or flour. Every 6 or 8 feet drive a piece of 1/2 inch rebar into the ground about 18 inches deep and leaving about the same above ground.
Go to Home Depot and purchase 3/4 inch PVC water pipe. Depending on your widths cut the lengths necessary to make a PVC hoop from rebar to rebar across the span. Height to your pleasure. Cut as many peices as needed. Then glue them up.
I like to have a center ridgeline
to help strengthen the whole thing. And, if big enough one at midheight. This
means you also need "crosses" If you are in a windy area attach 2 X 4 pressure
treated wood at the ground and attach to the rebar. Then you can attach the
plastic cover to the wood.
Once you have the basic frame up and as strong as needed lay thin pipe insulation over all the fittings to minimize friction. Then cover the whole with greenhouse plastic...looks like ordinary visqueen. but is UV resistant and much more durable. Available from nurseries and such supply companies. I got mine from Peaceful Valley Farm Supply. Also buy the clear tape for repairs and to put it all together.
Lastly, roll up 1 X 2's at the
edges so you can have an openable area for ventilation. And as a way to attach
to the 2 X 4's at ground level. I also put a door at each end.
There you have it. I think the total cost was pretty minimal. Most expensive item was the greenhouse plastic, but very much less than a prefab greenhouse.