I love this idea! It appears to be incredibly simple, and all I need is the wood spacer and the heating element. My intent is to use the gravity filling cups even during the winter; I have been wondering how to make that work when the temperatures lower. I would note that I initially didn't see what kind of heater it was (I had to recheck the pictures); perhaps make a more obvious note for people like me.
As for the heater, I didn’t mention a name because I’ve seen the same heater from multiple sources/brands. I have since found a similar heater made FOR plastic waterers, but if was $85 - too much for my budget.