I think PEX would flex too much. It holds its shape, but I don't think it would be rigid enough to keep the "hoops" in proper shape.
If you get the shape you want, make a large jig that will hold the PVC in place. Get a big old pan, fill it will sand and heat it up. Tape one end of PVC tube, fill with hot sand, then bend in jig to shape. once cool, pour out sand and PVC will keep that shape. The PVC can get brittle, but it will be under less stress if bent into shape with heat then cool versus just bending it and making it stay in the shape by force.
We don't heat to bend our PVC, we just bend it cold, after it is bent in a "hoop" for a week or so, you can take it down and it keeps it's shape. I have some PVC hoops laying out back right now, they've been there for a year now and the are still in a "U" shape.
We had some PVC hoops come unhooked during the wind in our greenhouse (the one pictured on the first page), the pipe moved less then a foot away from where it needed to be, I just hooked them back on and it's still holding great.
The biggest thing is to use small rope and pull it tight between each hoop, this will hold your plastic tight and it will not flap in the wind. We also hook our greenhouse plastic on by nailing the plastic between 2 boards and then pulling the plastic back up and sandwiching it between another board so it holds tight. if it's just between 2 boards then it will pull through (I know from experience)
Edited by BTS, October 29, 2015 - 11:13 PM.