If you live in an arid area, you can take an unglazed clay pot, fill any holes in the surface (like the drainage hole in the bottom), and invert it over the things you need to keep cool. I use the water catcher base from a much, much larger pot as the base and the lip of the inverted pot fits INSIDE of the base. If you keep the outer surface moist, evaporation will bring the internal temperature down somewhat. The base will catch any water that spills off the sides as you are soaking the pot and form a seal to keep the cold(ish) air in and the hot air out.
I've used this method out at the farm to keep some stuff cool. I wouldn't actually call it "refrigeration", but it does keep the contents somewhat cooler.
Every time you lift the pot, the accumulated cold air spills out, so you can't open it very often. Once or twice per day is about right.
I've had a 12 v cooler and it was much, much more efficient than the clay pot. But it only lasted a few trips. The pot is probably 10 years old or more and still works fine.
Thanks. I’m going to try this even though I’m not in an arid area.