That is a question I have thought about over the past year and I could not really think of anything else. Last week my freezer tore up must have been out 4 or 5 days the way things looked when I opened it after seeing water on the floor. Should have thought about doing that long ago.
Just throw one (or two) of these in the freezer. They “report” low battery but keep on working.
https://www.ebay.com/itm/278107972622
Visonic MCT-340 Contact Sensor
Most contact sensors, motion sensors, leak sensors, etc., that also report temp can double as temp sensors in fridge/freezer. I have used Samsung & Centralite leak sensors, Iris motion sensors, Visonic contact sensors, etc., to monitor temps in my fridge/freezer. Good use of older sensors that stop reporting primary status reliably (contact/motion/leak) but still report temps.
Though to confirm, am I seeing that correctly, hole drilled through fridge to run cord for sensor inside? Mad props for testosterone-enhanced monitoring! 
I have lots of these but felt better drilling thru the door. Not sure how accurate the contact sensors are and know i have a good connection to the network.
Seems counterintuitive, a fridge should be more like a Faraday cage, but I've never had any of my fridge/freezer sensors fall off the network or not report.
I do like the visibility of the number right on the fridge, that is kinda cool.
That’s why I use two of them. I calibrate them in a plastic bag in ice water. Once they’re in the freezer they both show -4.6°
I have a Moes temp/humidity sensor in a Ziplock bag in both my fridge and freezer. They of course due to the temps show the battery at sub 5% at all times but they keep on working. Never drop off the network and never fail to report. I am suprisingly impressed with each to be honest. As @danabw said it does not make sense with them inside a steel box but as many things they defy the laws of logic. I am impressed however with the drilling through the door.