I am using the one Pete recommended and it works great.
A word of caution. Drilling a hole in the side of a freezer is risky. There are freon lines snaking through the whole thing. I gave it a lot of thought and ended up fishing the wire thru the condensate drain in the bottom. Mine is not frost free. I am not sure if frost free freezers have a drain.
This sensor will measure down to 0 F. I have my alarm set at 10 F, and it has already gone off a couple of times when I left the door open too long. I am glad that I have it.
I use CAO wireless Tags. About 1.25" square by 1/4" thick you just throw them inside your appliance and they send data to your Tag manager.
Cheap, easy and takes a CR2032 cheap battery about every 7-15 months. They include many ways to send Ethernet data to anything, including push notifications, REST, kumoApp (js) programs, email, text messages etc.. Comes with temperature, humidity, lux, moisture, battery voltage, online charting etc.. Costs about $25 per tag plus one manager $30??. I use many of them. Accurate to about 0.1 C and resolution to 0.0001 degree.
They communicate through metal boxes, like freezers, fridges and I have detected on in my car about 1.5 miles away. Told me my wife was coming up the hiway. I live in the rural with low RF noise and very few buildings though.