When I was in MA I used the heated dog bowls, similar to this. It's been almost a decade so I don't remember the brand, but lasted for the life of the chickens, 8 yrs:
Farm Innovators Model P-60S 1-1/2-Gallon Round Heated Pet Bowl, Speckletone Gray, 60-Watt https://www.amazon.com/dp/B001G1C8BQ/ref=cm_sw_r_cp_apa_PKeWzbW1RHD54
Farm Innovators Model P-60S 1-1/2-Gallon Round Heated Pet Bowl, Speckletone Gray, 60-Watt https://www.amazon.com/dp/B001G1C8BQ/ref=cm_sw_r_cp_apa_PKeWzbW1RHD54