Freezer Temp sensor

I live in a rural area and we store a lot of meat and food in freezers in multiple outbuildings across our property.3 freezers next to each other in one building and 2 in another. Over the year’s we’ve lost hundreds of pounds of food due to freezers going out, kids leaving the door ajar and power failure.
I would like to install temp sensors inside these freezers and setup alert notifications if the temp gets too high indicating that there is a problem and hopefully saving money in the long run.
I’ve used Rpi3b in past projects. Does anyone have any other hardware and/or configuration recommendations? What kind of sensor should I go with?

the best thing for your case is ready to use out of the box product https://mydevices.com/.