Do you live in snow country?
If not, you can use coil up black plastic piping or copper painted black but if snow is all over it then it is not necessarily going to work. When I lived in California, people used this stuff to heat their pools and coiled it up on the roof. You could do something similar on the top of your coop - I would need more info and pictures of your coop. Also you could use a 5 gallon bucket of hot water with a lid - someone else posted a similar thing - to heat your coop with out electricity - Put hot water 200 degrees in a 5 gal bucket or something similar - tighten the lid and it will keep the surrounding area warm as it cools during the day - you can put your waterer nearby. inside the coop = you just want to keep it from freezing. other things that insulate - you could use newspaper or hay in and around the coop. again depends on your coop but both are good insulators. If the ambient temp is above freezing you have succeeded.
Caroline