Here is a very simple idea (don't know if it is the one you meant) to keep things cool. One pot inside the other, with wet sand in between - see itdg.org/docs/ agroprocessing/FC29_34.pdf.
Karen Hampson

My Gran used to have a device that she kept her milk bottle under. It was like an extended flowerpot without the hole and it sat in a dish of water. The theory was that the terracotta absorbed the water which then evaporated, cooling as it went.
Dave May