Low-tech ingenuity — cooling by evaporation. The outer pot should be a sealed water jar, the inner pot permeable. Keep the sand wet, but not overly wet. Perfect for tomatoes, they last 3-4 times longer than on the window sill in late August.
actually only the inside pot can be glazed, the outside one could be unsealed as that is the one absorbing the water / evaporating.
i do have a question though if the idea is to cool the inner pot why would you have it made of clay not something more conductive to cooling like metal or something possibly copper or aluminum (cheaper)