A cellar welded from sheet steel with a thickness of 6 mm (this is the minimum permissible wall thickness) will last you a lifetime, you only need to provide stiffeners so that the walls do not bend with soil. I did country a cellar 180 cm high, 160 cm long and 150 cm wide (see diagram). But you can resize as you wish. The hatch through which you get inside the attached wooden stairs can be arranged anywhere (side or center), but the height is at least 70 cm.
Of course, a specialist should weld such a country cellar. My friend helped me out the garage - a professional welder. When the iron box was ready, the question arose: how to drag it into the garage? We made slopes of small logs, took the usual construction scrap, which I and my partners used as a lever (see photo). Mounting handles were prudently welded to the box on all four sides - this made our work easier.
Before lowering the box into the prepared hole, I processed the outer surface of the walls and the bottom with bitumen. He warmed it up and added a little diesel fuel to it for greater plasticity. It was with this compound that I coated the insulated surfaces with a large brush. Waterproofing is. Now we need to think about a heat-protective layer. Its thickness around the box should be at least 70 cm. For these purposes, I took slag, but if it is possible to backfill with wood sawdust, then the latter is preferable.
Metal, like any other, cottage cellar needs good ventilation.
I arranged it like that. Knocked out two exhaust ducts of a small section (10x10 cm) from edged boards. One box should be higher than another. Set them at the opposite ends of the pit. The lower end of one of them is located closer to the bottom.
He made shelves for vegetables and the bottom of the boards so that the vegetables did not touch the metal.