My step father found some pretty heavy duty resistors in the garage that put out alot of heat. We have drilled them into a huge heatsink (30cm x 15cm) and attached it to a board which will go on the side of the kennel like a radiator or wall panel heater.

As the temperature in the kennel drops below 13 degrees, a variable temperature controlled circut raises the voltage of the power going through the 3x resistors on the heatsink, heating it up like a radiator.

Its 12v and uses 50 watts of power. So around 12 degrees airtemp it puts about 3 watts through, as it drops to 5 degrees in there, it puts 25 watts through and as it gets to 0 degrees, its running at full capacity at 50 watts. Basically the closer it gets to 0, it will put through more energy.

However with the radiator heatsink on the side wall of the kennel and sensor next to it, it wont ever get down very far. The goal is that it wont ever drop below 10 degrees - the radiator will balance it. We are putting some insulation in a box on the floor of the kennel so he sleeps slightly above the door as heat will get trapped in the top of the kennel so we put him closer to the heat.

With the sensor in the freezer, the heatsink near its heat points / resistors gets too hot to touch.
With the sensor in the fridge, i would describe it like a natural hot pool temperature - too hot at first but you realise you can keep your hand on it and get used to it.

Best of all, he cant chew on it to damage it but he can lie with his back on it if he wanted. The transformer will be in our garden shed, with a couple of extension cables (so we have a thick wire gauge and no voltage loss) carrying the 12v AC power 3m to his kennel in the garden. At max heating or 0 degrees, it draws 5 amps.