your heater should not rest against anything, plastic or otherwise. Use the clips that should have been supplied with the heater to attach it. This will ensure that there is enough water flow around the entire diameter of the heater. This will keep the heater coil from being damaged, i'm not so concerned about melting the plastic, i don't think it would get that hot.
I would be inclined to say... probably not the best idea... I think you'll want something with more direct flow for the heater. of course, i'm not all that familiar with the CPR skimmers.