I use a regular wet/dry heating pad under the developer tray in the winter, when I keep the house at 53F. I bought the pad at the drugstore for $12 or some such. It was the smallest lowest wattage pad I could find.
I put a towel under the heating pad to keep from warming the darkroom bench and another over the pad to limit the amount of heat getting to the tray.
The result works very well - the temperature stays constant even though there is no thermostat [which it should if the ambient temperature is constant].