Have you tried a bird bath heater, like the one in this link, to heat your plastic waterer? It says it is safe with plactic, copper, concrete.
http://www.birdbaths.com/bird-bath-accessories/bird-bath-heaters/150wattbirdbathheaterdeicer.cfm
http://www.birdbaths.com/bird-bath-accessories/bird-bath-heaters/150wattbirdbathheaterdeicer.cfm