hi just a thought . you can buy from auto parts stores empty barrels used for antifreeze or different oils that are blue plastic. Blue absorbs the sun s rays great, not as good as black but good nonetheless. Get some of those barrels ( 5 - 10 bucks,) wash them out and find some small ( 8 year old's fist sized) river rock and fill the barrel with them. Place them opposite either a glass or plastic south facing window and that barrel at the end of the day will defy you to hold your hand on the barrel without being warned off with the heat. that barrell will emit heat all night. and u will find your chickens hugging the barrel at night.
I also use the earth as a berm against the walls. at least 4 feet thick and high . Placed on the north and east walls will insure a 45-55 degree heat sink in your coop which would be very acceptable to the residents during a 20 degree night. IF u have good insulation in your coop, adding a propane light ( encased in a good hardware cloth 360 enclosure) with an empty coffee can on top produces 2000 btu's of heat and is quite effective in warming up the area. there are 90,000 btu's in a gallon of propane approximately, so judiciously used, it will not be cost prohibitive.
good luck and remember to have fun doing it.