you could run a length 1 inch by 1/2 inch wood battern along the length that your planting climbing plants, then screw closed up hooks or circles that have a screw thread into the batten. then run your strings up through the loops/ closed up hooks.
personally i don't like the idea of poking galv wire past the polythene, and the polythene resting on it. i think it i would prob tear it while doing it and the polythene resting on the galv wire may promote hot spots.
if i get chance at some point i'll take a picture of something similar i've done.