I built PVC hoops for winter growing and made my own clips out of some kind of flexible tubing. I don't know what it's made of, but it was near the conduit (I used electrical conduit rather than plumbing PVC because it's UV-resistant). I just cut the tubing into pieces several inches long, then slit down the side so that I could slip it over the PVC pipe with my sheet plastic in between. It would've been really cheap (but labor-intensive), but the employee couldn't get the tubing to scan and actually gave it to me for free.
My homemade clips work well on the ends of the rows, where the plastic goes around the last rib, but tend to pop off the mid-row ribs when there's a heavy wind to puff up the plastic. I just use them on the ends, and no clips along the rest of the structure, and have no problems.
Here's a photo. Not sure if it clarifies things, but you can see my little clips near the bottom of each PVC rib. I've since switched to weighting down the end of the sheeting instead of using so many clips, but don't have a picture of that.
http://cubits.org/pics/2011-04...
