Chris Hogg was thinking very hard :
In which case a simple temperature sensor might not register the
problem if the temperature doesn't rise far enough. Perhaps a
microswitch on the door, connected to a time-delay system, allowing
you a few minutes access to the freezer before the alarm goes off?

As said, it works fine, responding quickly to the door being opened. It
generally shows minus 17.5 to 18.5C, alarm set to trigger at higher
than minus 13.5C. Left open for a full minute, the alarm sounds, but
easy to ignore if you wish to, because it only sounds once per minute.