Two nested unglazed terracotta pots with a roughly 4cm layer of wet sand packed between them; evaporation of the sand's moisture draws heat out of the inner pot, dropping its internal temperature about 10C below ambient under good conditions (dry air, shade, airflow, sand kept moist ~2x/day), extends most vegetables' usable life by 15-20 days over open air, per the source. This is the active-evaporative mechanism that sand-box root storage explicitly does not use (that method separates and buffers produce but claims no cooling effect). Shares evaporative cooling's core limitation: effectiveness drops in humid air, a real caveat for Normandy's oceanic climate, so this is scoped as a supplementary short-term buffer rather than a primary cold-storage solution here.
Note: Cost, carbon and footprint are rated against the other options for this same need, not in absolute terms.
Shaded, ventilated siting; a dry-air location improves performance, flagged as reduced effectiveness in humid climates
No sizing or quantities: how much a technique yields, and how much a household needs, both depend on the build and the people, so the catalogue compares options rather than dimensioning them. It carries no bill of materials and no build sequence, and it is written for an oceanic, clay-soil climate (Normandy, France). Regulatory notes are indicative and need confirming locally.