This hint was from a Peace Corps Cookbook. From here. This sounds possible to do with the 5 gallon buckets that I will be using as hauling and washing containers. And if I am headed to the desert there will be plenty of sand. There will also be shade with the the tarp.
A makeshift cooler: Put items to be kept cool in a clay flower pot with a sealed bottom hole (I used duct tape, but a cork will work, too). Locate another clay pot that is 1 inch larger all around than the first pot. Seal the bottom hole shut on that one, too. Now put 1 inch clean sand in the bottom of the larger pot, place the smaller one inside, and fill around the small pot with more clean sand. Wet down the sand well. Cover both pots with a wet cloth. Place in a breezy spot (preferable) in the shade. The temperature will go down quite a bit from the water evaporation. I made one to keep my butter from melting, and when it was 98 degrees in the sun, the cooler (in a breezy shade spot) was only 75 degrees. It worked. It did not get ‘refrigerator cold’ but it kept milk and butter from spoiling. The amount it will hold is limited by the pot sizes, but it will keep the butter from melting, and veggies will store longer inside one. Just keep the sand watered down. This can be quickly made in a camp and town down if you need to move it. The only down side I found is if you drop the pot it will break. (Huge ones are made to keep vegetables in some parts of Africa where they are sometimes called Zeer Pots.)