I just use an aquarium heater that is fully adjustable in temp cut a hole in the lid with a holesaw the size of the bung you have brought cut the plug of the lead and push it thru the hole in the bung wire a new plug on the lead and Bobs your uncle
Sticking my hand up as a brewer but i dont think im in your league here as im still a kit brewer more so as have never had the chance to meet someone to help and show me the way
Rowdy Rod