I’ve just done this. Tube heater was 14 quid and the inkbird was 26 quid. Both from eBay. I’ve got a hoppy pale fermenting in there now. The tube heater is 1ft long and kicks out 40w. I took the plug off the heater and ran the cord through the drainage hole in the fridge. The temp probe just goes through the door, so no drilling involved.
Ive never needed a fermentation fridge before as the temps in the room were always stable but some changes at home (the wife needing more space for her hobbies
) means my last brew went down the drain as it went into fermentation just as the hot period hit us.