Check out Aqara water leak detectors. They have two external contacts that when they are bridged change the sensor state to ON. You can run wires to those contacts.
Surely the best approach here is a temp sensor with a threshold at which to alarm? Then you're actually monitoring the state of the appliance, not the state of the door.
While that is a solution and solves a big issue, especially like a power outage, a door sensor stops this problem long before that is an issue. My freezers and fridges are on the whole house generator, so power problems don't effect them. A temp sensor is still a valid thing long term but does not solve the problem I am trying to solve.
I second this... it also has the advantage of alerting you if fuse pops / someone's unplugged it / whatever reason.
There was a recent discussion about this very topic recently...
Open the door sensor, locate the reed switch, remove it and connect wires to its PCB pads. Although it should be pretty straightforward to measure the minimum distance required between the magnet and the sensor and install the magnet just a bit closer, so that any cracking of the door opens the switch. If you use a weaker magnet, the sensor will be more sensitive to the distance, so you will be able to detect smaller gaps.
Side note: if the door doesn't close when there's a small gap, the fridge might not be level or your door auto-closing mechanism might be worn out.
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