A word of caution here. Years ago, I had week old baby chicks when the power went out. No problem, I thought, I'd just fill some milk jugs with hot water and place them around the brooder.
The hot water filled bottles put out a lot more heat than I thought they would. It was a very good thing I was as doting on the chicks as I was because I saw that the chicks were beginning to pant and otherwise show signs of heat stress.
In my single-minded focus to provide heat for my chicks through a power outage, I almost killed them with too much heat. I quickly removed all but one of the water bottles, freeing up three-fourths of the brooder to be cool.
Since then, I've learned that only a single heat source is necessary. The chicks will warm themselves when chilled and then move away from it. It's always better to err on the side of having a brooder on the cool side than to over-do it with too much heat.
Do you have an old fashioned rubber hot water bottle? I've used one of those in a power outage when I was brooding chicks under a heating pad. The hot water stays a lot warmer for much longer and the chicks can snuggle against it if you wrap it in a fuzzy towel.
Oh, I thought of one more thing. If you boil water for the purpose of providing heat in a brooder, you are risking burning the chicks. That water is 200F, and you only want water at around 90F, so make sure the hot water is not so hot that you will burn your hand if you placed it against the bottle.