I use a small stock tank heater rated for plastic too.
I am far happier with it than those heated dog water dishes that give mice a toasty warm winter lodge then fail likely due to mouse activity.
I should have called it a deicer.
I use a 250 watt stock tank deicer in my 5 gallon bucket. For use in containers up to 25 gallons, including plastic containers. Put the deicer on the bottom of the drum, to keep the nipples from freezing.
I'm gonna make another 14 gallon drum and use that instead of the 5 gal bucket for winter. I need a bigger heated waterer for when I go away for a week when it's colder, since I have more chickens now.
https://www.amazon.com/dp/B001E8X2CA/?tag=backy-20
Your post reminded me of something important.....
Deicer <<< that is what you want.
Heater <<< many are actual heaters that heat to scalding.
He sure and read carefully so you get a deicer not a heater.