Personally I would say just get an Inkbird. It has the thermometer on a wired probe and can control the heating and cooling for a constant temperature. In my opinion its a no brainer, even if its about ��ã30 depending on which model you get.
I've got a STC 1000 (you can get them for under a tenner) that you have to wire up yourself but by the time youve paid for a box to wire it into, cables, plugs, etc it really isnt a lot cheaper than an inkbird anyway. I'd definitely recommend the ready made Inkbird instead, there are plenty of threads on here about temperature controllers. If you go that route you could easily add a cheap heat source, some on here have reptile/aquarium types, again lots of threads on here. The only thing I would say is if its too well insulated you may find it gets too hot. I have a heat belt controlled to come on when the temperature drops. It sits under a blanket in my garage and even when it drops below freezing outside it maintains the temperature really well. Overheating is not a problem at this time of year. I also have a larder fridge (free from freecycle) that I will be setting up to add cooling for the summer, which will give me heating and cooling controlled by the temperature controller.