IMO, if you are powering a device from a cranking battery you need a cutout no lower than about 12.3V if you are worried about the lifespan of the battery.
If you are powering a fridge - most have low voltage cutout already. My Weaco has 2 cutout settings - the higher one is 11.7V and that is still too low for a cranking battery.
Also, do you need a socket? IMO power sockets don't belong in the back of cars if they are for semi-permanent connection. Even ruggedised power sockets seem to get loose or are easily knocked out.
IMO use an Anderson plug or if you are looking for a cheap solution - even insulated crimp connectors which is what I have for my fridge in the Vit.
I use figure 3 and figure 21 for the fridge in the Vit with the terminals arranged so the battery +ve side is female, battery -ve side is male and the wires are held together near the terminal so the battery side connections can't be connected into each other.
