[Alan1]

I use a round plastic Keter tub (was £20 in a sale but are £45 full price) with soil in it in the greenhouse (but would do in a garage or shed) sitting on top of a piece of 50mm thick Celotex insulation board and insulated with a tank jacket and cheap duvets and more insulation board so that the temperatures are fairly stable inside, it insulates it against getting warmer as well as colder. The tub has a panel removed for a doorway but is covered apart from a small bit for air. Once it gets warmer again some of it can be removed to allow temperatures to gradually rise.
I have a wireless thermometer sensor (about £30) inside the tub to monitor the temperature which I can do from the house. In case that fails I have a £2 thermometer, the type with a wire probe attached and with the wire hanging inside the tub which I'd have to go out and read until I got a new wireless one. Neither of these are under the soil so I know it will be more stable under the soil.
I check the temperatures on the weather forecast every day for a few days ahead.
You'd not want it where rats could get to it.
People use the fridge method and I've seen them say they fill bottles of water and put them in the fridge as it helps keep the temperatures more stable than if it was empty. You'd probably have to open the door once a day to allow air in. The smaller the fridge the harder it is to keep temps stable. I know that my torts move around in the soil during hibernation, as much as a couple of feet so for that reason I personally won't use the fridge method but many people do.
There are people who keep their Horsfields going all winter with the heat lamps etc but I think they still slow down and are looking to hibernate