Can you use one of those oil filled radiator heaters. Very safe, and provides a fair amount of heat (more than a radiant panel). I'm not sure of the dimensions of your coop, but one placed somewhat centrally should at least raise the temps for all but those on the ends of roosts close to exterior walls.
Are your coop walls insulated? I have done that with my original coop, and it was pretty inexpensive. Used the 1.5" solid foam sheets (I built mine with the 2X4 sideways, so ~ 1.5 inched between exterior and interior walls). I opened up old grain bags & used as 'house wrap' before putting on exterior plywood, and used the really thin plywood for the interior. Used the thicker solid foam sheets under roof, as those 2X4 were on edge for stronger roof.
I do still have lots of ventilation, but my coops are small compared to yours, so if I had electricity out there, a panel heater for each coup would be sufficient.
If you could at least insulate 2 adjacent sides, and put the heater towards that corner. I'm guessing they would choose to roost closer to the heat on really cold nights. It would ultimately be safer and, probably, cost same or less to run the heater.
I got one of those oil filled heaters at a yard sale for $5 a few years back - and it was practically brand new! Love it when I need it in the house (when in-laws come it goes into their bedroom for extra heat.)
![IMG_2273[1].JPG](data:image/svg+xml;charset=utf-8,%3Csvg xmlns%3D'http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg' width='2000' height='1500' viewBox%3D'0 0 2000 1500'%2F%3E "IMG_2273[1].JPG")
Hmmm, wonder which nipple water tastes better????
