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I'd be wary about drilling through the back or sides, too easy to hit a refrigerant pipe. I just slightly enlarged the drain hole, cut the plug off the heater to pass the cable through and rewired. The Inkbird thermometer probe when straight through.
I left my fridge stat in place, just turned it to max. But I want to be able to sell the fridge again if needs be so didn't want to do anything drastic.
Mines 40W. I wouldn't worry about the power, just make sure it's horizontal, the inkbird with take care of the rest. It's surprising accurate when set up.
Are you bringing the cables through the condensate drain at the back?