Completely saturated salty water freezes right around -6F or -21C. And most home freezers are set to right around that for energy conservation reasons, but max out even lower. So you could use any briny water and it will phase change before your food melts.
I'm surprised a 12v based deep freezer that runs off a reasonably sized solar panel is not more popular/affordable. With the right amount of thermal mass, and good insulation, and a small battery you could greatly extend how long your food stays cold. There are a lot of people who live in places where snowy weather or high winds and loss of power is an eventuality. In an emergency losing your food is a bigger concern than just the cost of the food.
Or go a bit higher end and put a larger solar and battery instalation with an inverter that can power the whole house. As solar panels and lithium batteries come down in cost that's becoming increasingly attractive for rural areas where the grid is flaky the best of times.
12V/120V freezers do exist and are marketed towards RV, camping, and off grid markets. Disadvantage is they tend to be smaller, cost more. But they are very efficient. Where a residential freezer might draw a couple of hundred watts dual voltage fridges draw 25-50 watts. And automatically switch between 12VDC and 120VAC. So they can be connected to both shore power and a battery at the same time and it 'just works'
There are also propane fridges/freezers but they are also smaller. (Though at the point where you have a large propane tank, you could arguably just as well have a backup generator.)
My impression is the big advantage of absorption fridges is they will run off a tank of propane for a long time. But Downside they are very slow to cool down and running off AC/DC they aren't very efficient.
But yeah cheapest thing to do is get a furnace transfer switch and a generator. That seems to be how rural people in the upper midwest avoid freezing to death when the power goes out in the winter.
I'm surprised a 12v based deep freezer that runs off a reasonably sized solar panel is not more popular/affordable. With the right amount of thermal mass, and good insulation, and a small battery you could greatly extend how long your food stays cold. There are a lot of people who live in places where snowy weather or high winds and loss of power is an eventuality. In an emergency losing your food is a bigger concern than just the cost of the food.