If you want you could also put straw balls along the perimeter of their enclosure, they been found to have almost the same heat retention as insulation, so it can help block any drafts and could give them an extra few degrees if they decide to snuggle in. I've had ducks in their coop at -40 windchill, and as long as they had access to a draft free corner with lots of bedding, they've kept each other warm. Another thing you could do if you chose is feed them high fat treats before bedtime, like scratch grains, unsalted peanuts, or something like that. Processing the food or treats helps them (any poultry/fowl) keep a little warmer over night as well. People in my area choose to switch from scheduled feeding to free access feeding for the winter for this reason, I just keep an eye on my overweight prone animals when I do this (I have a jumbo Pekin). I'm sure you can also Google other treats high fat as well. Just make sure they have access to unfrozen water if you do decide to do this...
And if you do want to use something to heat them, I've used wall mounted heaters like what they sell at tractor supply for chick brooders as they have no exposed heating elements and also waterproof feral cat house warmers (slipped under the shavings of their nesting site). They don't need to be toasty, but sometimes even a few degrees can help make a difference, especially in the extreme cold.
