That's something I've been thinking about doing.I built a small cold room. Insulated walls and ceiling. Size about 3x6 by 8 feet tall. Ran a 6 inch pipe to outdoors for a bit of cool air. Vent outlet in ceiling. It probably gets to10 -15 C, cooler in Winter. Light and door. Better than nothing. It keeps garlic, potatoes and onions reaonably well.
We have two options. We use one small bedroom as a food store. It is South West facing so would get very hot in the afternoon. But we've set up shelves in the window and enclosed it all in insulated silver fabric. So essentially it creates a nice sunny grow room in the window but the rest of the room stays cool. Our generators are in this room (which need to stay cool), as are our freezers - so that makes them more efficient to run. But it tends to hover around 17 degrees C all year.
We also have a big walk-in cupboard in our utility room that we could insulate. It has no windows and the only external wall is north facing.