Depending on your budget and what you have access to really. You can start with wrapping the incubator in a towel or blanket for short term. Hand warmers are pretty brilliant, they last for hours depending on which one you find, only thing is there's kind of a curve to the heat, very hot and then it slowly cools off, I actually but them in bulk for transporting chicks to buyers. If you have enough spare money you can purchase a battery back up source, they can be charged in your car if needed as well. On that note you can also get a plug converter for electronics for the "cigarette lighter" DC power points in your car as well. Is your stove gas by any chance? Heating well cleaned stones would work as well.