Thanks for the update! Your fruit cellar could almost be likened to my boxes, except my boxes get 10-15°F colder. I do find that plants overwinter better in the boxes than in the unheated garage. When we deal with temperatures colder than what would be normal in their natural state, I think the smaller temperature swings during the winter is important.
Rick Rodich zone 4a. Annual precipitation ~24 inches
near Minneapolis, Minnesota, USA