Do you mean plastic as in greenhouse type plastic that is tightly stretched over the rafters? If so then yes you need something to stop it rubbing and to stop hot spots degrading your plastic. If you are talking about loose plastic somewhere else, I have no clue.
I'm guessing these are 2x4 rafters, so you'd be spanning probably 1 1/2" (sorry, don't have the cm off the top of my head.)
So 2" PVC pipe cut lengthwise, whatever would span the top/end of the rafter, placed between the plastic and the wood, and screw right through the PVC. You might have to drill a starter hole, but it's very stable, paintable, lasts a long, long time in the sun, longer than the plastic will. You can buy the thin stuff, if it's available. That is not to code anymore so it's harder to find, but PVC is not that expensive. Take into consideration the movement of the plastic in the wind, make the length of the PVC long enough to protect maybe even the underside edge from rubbing against the wood in the long run. You could even use 45 degree or 90 degree connect pieces, cut lengthwise, glued to the pipe to turn the corner if you want to go down the vertical part of the wood.
I slide whole PVC pipes around the rebargreenhouse ribs that make up my greenhouses.
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