Although I haven't used this option, there is a cheaper option than a laptop heater. Pet stores sell pads to put on the floor of an outdoor doghouse, that put out anywhere from 15 to 60 watts, depending on the brand--just enough for an outdoor dog to lie on to stay warm. One can also buy a Thermo Cube--basically, an electrical outlet that turns on with thermostat control--for under $20 to turn on a pet heater or any other heater only when the temperature is under 35F (Thermo Cube sells several variants for different temperatures, including one that turns on when it's too hot). It should be possible to buy a dog (or cat) heating pad plus the Thermo Cube for $60ish.