I use the original red heat lamp [with clamp] that I used when raising them with a daily timer plugged into the extension cord.
Here in Missouri last year we got snow a good six times and it was 2-5 inches each time.
I also put a thermometer in the coop [4X6X5] and it kept the coop a cozy 55-60 degrees when it got to 0 degrees F with windchills below that.
I like the heat lamps red glow as it keeps them calm at night. We noticed our chickens could not rest as well and were more agitated with a 'white' light.
Crazy thing too. When we pampered them this past winter like this we had 18 of our hens laying 12-7 eggs a day. It was awesome!
Much more fun than the 1-4 eggs we were getting from the same bunch this summer when it was 100+ for 45 straight days...
I'd suggest a red-heat lamp light in a clamp-style light fixing over anything.