I used a gallon-sized water container here on the Western Slope of Colorado when the nightime temperature dropped below zero and I think it worked fairly well. I put almost boiling hot water in it, topped it off with a bit of hot water from the tap, took it out to the coop and buried it in the shavings. Completely to the little blue cap. I put it out around 7:00 pm and when I checked it at around 6:30 am, it was frozen, but not much. There was probably less than a quarter of an inch of a frozen layer on the top and sides and the hen house felt warmer.
My coop is 4'x6'x 4.5' high and is well insulated. All but one vent is covered by laying vinyl over the openings and I have six standards so they help keep the heat in.
I think the trick is to bury it in the shavings so the heat is released at a slower rate. I think it worked and I'll use it again if I need to which might be Christmas Eve, I hear.
Mary