What do you mean by "pvc railings?"
I have about 12 railing planters placed over both wood and composite decking for 2 years and it is fine, no marks. I wouldn't put down plastic as it can block air circulation and cause moisture condensation.
The weight of the soil holds mine down just fine. I only lost one when I left a large table umbrella up in high winds, and it flew over, knocking out and smashing the planter.
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