Here americangamefowl describes many different kinds of root cellar and the technical aspects of effective root cellars. There is really rich information here.:
DeanLeatherman explains and shows how he built effective root cellars for root vegetables and cabbage from large plastic barrels buried in the ground. He talks a bit fast, but explains his concept and what he did well.
I did wonder whether the plastic would be secure enough storage against any burrowing animals, but he didn’t mention rodent infiltration as a problem he’s experienced so far.