In my experience a plastic waterer does work but the heater definitely needs to be metal. A metal watered works better because it will continue to conduct the heat & keep the water from freezing. I have one I made (link in my signature line) that has worked through -20F winters. It does need a...
I just made a page of the ones DH helped me make, they have been working great and well worth the little time it took to make them!!
https://www.backyardchickens.com/a/dhens-water-homemade-water-heater