Hey Macca,
lots of different options and they all work well.
I used a heat belt for years and it was very effective. I wrapped it around the fermenter about 1/3 of the way up from the bottom and had it plugged into the temp controller.
A good mate uses a reptile heating lamp (I.e. a lamp you’d use if you have lizards/snakes) And swears by it. Another option is an aquarium heater, which you suspend in the fridge.
worth remembering a fridges holds temperature pretty well and the fermentation will generate heat which will warm the fridge as well.
JD