I got one of those basic Xiaomi Aqara sensors rated to -20 but actually its down to -22. Thing is I’m only interested if the temperature starts rising towards 0 for an alert. So suits me fine. Unless you want high accurate freezer monitoring for some scientific project or curiosity I suppose 
Been there, done that with the same results. I hadn’t thought about mounting a contact sensor there. Thanks for sharing. I’m going up see if I can’t do the same thing.