This is going to sound a little overkill, but I do have some experience in this area, and this is the right forum for it so... Digital thermometers for freezer

This is going to sound a little overkill, but I do have some experience in this area, and this is the right forum for it so... Digital thermometers for freezers are relatively cheap (if you don't need them NIST traceable, anyway) and easy to get from any number of laboratory catalogues (See, for example, Thermo-Fisher or Cole-Parmer). You can even purchase a chart recorder that'll sit on top of your freezer if you want! If you're just concerned about brown-outs and fluctuations, you can go with a cheap, magnetic max/min like the Cole-Parmer Jumbo (fine for consumer freezers) that sits on the front of the unit. You need to thread the wires through the freezer door, though, unless you're lucky enough to have a port on the side or back. If you want a boatload of data you can go all out with a digital datalogger with a Type T thermocouple (Type T will even work reliably on -80C ULT freezers). I highly recommend Onset Hobos for reliability. There are also bluetooth and wifi models that allow you to get the data to your devices. As an aside, stay away from Oakton as a brand if you're doing anything mission critical. If you don't want the wires sticking through the doors, I've had good success mapping temps with the Thermochron ibuttons - they are the size of a watch battery and some models work down to -40C. If you set the frequency low (e.g. every hour) they'll log data for months before you need to pull them. You can also get waterproof enclosures for them and throw them in the dishwasher for fun!