Progress on the Cold Frame / Low Tunnel

The weather this past weekend was so beautiful N and I couldn't NOT get work done outside.  Saturday was awfully windy, but the sky was clear enough to encourage me to venture over to Community Forklift to search out supplies for my low tunnel, while N and R set off in search of nice trim to finish his workbench.  Sunday was warm and sunny as well, and I made serious headway on the low tunnel construction, in addition to cleaning out my shed (woman-cave) and clearing the platform behind it where we shove things like bamboo poles and excess mulch.

I was able to find enough 1.25" PVC pipe and fittings at CF to build a neat little frame over my cold frame bed.  Since I knew the pipe wouldn't bend sharply enough to form round hoops that narrow, I cut straight 2 foot sections of pipe and joined them together with L-bend fittings to make 3 rounded square arches.  (I did not bother gluing the pipes and fittings together, as they seemed pretty secure.)  N helped me drive bolts through the bases of each arch to secure them to the pipe sections I drove into the ground several weeks ago.  Then I draped some 1.5 mil plastic sheeting over the top, tied the ends closed with bread ties, and weighted the sides with rocks scavenged from my front flower bed.  I'll add large (1 gallon or 2 liter) containers of water as I collect the empties, to help stabilize the night/day temperatures.

R immediately went over and tested the security of the arrangement by swatting the plastic lightly with his hand, causing the center to cave in and the ends to ride up.  Hmm, not great.  With the memory of Saturday's winds still fresh in my mind, I immediately set to thinking about modifications.  I don't want to staple the plastic to the bottom of the frame for two reasons:
1) That would make it difficult to water, plant, harvest or vent inside, and
2) It would probably just rip away from the staples in the next windstorm (or toddler attack) anyway

So, the first modification I have planned is to lay a thin bamboo rod on top of the arches, along the length of the bed, and tie it to each one.  I hope this will provide additional stability to the frame while keeping rainwater from pooling on top of the plastic.  I'm not sure what to do about securing the ends, though.  I was thinking of adding bungee cords around the base, or those commercially sold clips that slip over the plastic and the PVC, but I'm reluctant to buy anything special for this.  (17ft PVC pipe and fittings = $9 at CF, not bad, huh?)  I could try cutting a larger piece of plastic and weighting it with more rocks, but I like the un-cluttered look of a neatly trimmed sheet.  Any suggestions?