Find, purchase, or make two pots from terracotta or clay. Use only unglazed pots, the sides must be sufficiently porous to allow for evaporation. Glaze on the inside of the smaller pot is okay. One pot should be large enough for the other pot to fit inside it and have at least an inch to an inch and a half space all the way around. If you found or purchased pot has a hole in the bottom, plug it with duct tape tape or even better use a 4” long bolt along with nuts and washers to bolt the smaller pot to the large pot and then seal with silicone.
Add an inch of coarse sand to the bottom of the larger pot. You want to create a base so that when the smaller pot sits on it, the top of the two pots are level with one another.
Once you have the smaller of the pots positioned on the base of sand, fill the space between the two pots with sand until you have only a small gap around the top edge.
Gradually add water, preferably cold, over the sand between the pots until it’s thoroughly wet and unable to absorb any more.
Cover the top of the smaller pot completely with a wet towel or cloth.
The smaller pot should start to cool down. You can use a thermometer placed inside the small pot to measure the temperature difference.
Place your fruits and vegetables or other items needing refrigeration into the smaller pot and replace the wet towel over the top.
Store the zeer pot in a dry place that is well-ventilated, which will encourage the water to evaporate . Placing it on a stand will allow for increased air circulation beneath the pot.
Check the pot regularly, at least twice daily, to make sure that the sand between the two pots is still damp. Add more water frequently to prevent the sand from drying out and to maintain the evaporation process which keeps the inner pot cool.