The cooling ability of zeer pots and coolgardie safes is relative. Their maximum low temperature under ideal conditions tends to be around 60-70ºF, which I'm sure seems cool when air temps in the shade are 100-120ºF.
I'm to really expecting real refrigeration but I'm thinking i could "over summer" some more of my potatoes in a contraption similar to this. just some large pots within larger pots in the shade with a little sprinkling of water in the soil in the exterior pot. not enough room in the frig and they just don't seem to hold that well in the ground. they either rot or burn up or get eaten by something.
It's a barrel that is laid on its side beside a building (north/coolest side), with a thick mound of soil over it, with the top now the door (insulated). It might be kept even cooler if it was planted with something (clover?) and dampened daily in hot weather.
no root cellar. sounds like the 'potato' barrel is basically the same idea. using evaporation to help cool the interior. I know where I can buy those big plastic 55 gal barrels. they will work great I bet.