I've heard of people using old vehicles such as vans or panel trucks, pulled into a hole in the hillside and covered with dirt, and leaving the back doors available to get in and out. We have a friend who buried a short-ish semi-trailer in their hill. It was being "junked", and a junkyard might be a good place to find a vehicle like that.
We also know of someone who got a huge walk-in freezer from a restaurant remodel and they managed to get the thing home and bury it for a root cellar.
My husband and sons hand-dug a root cellar 7 years ago. They chopped with a pick and shoveled the dirt out until they made a 6' by 8', 5 1/2' deep hole. Then they laid logs across the hole, overlapping a foot or two onto the dirt ground-level surface. The logs are about 10" in diameter and are laid one right next to another all the way across. Then they laid several wooden doors we found at the dump, across the logs. We covered it with a layer of 6 mil plastic, put about a foot of dirt on that, then another layer of plastic. Then we continued shoveling dirt over it until we used up all the dirt from the hole. The guys chopped a stairway down to it, and we made a frame on the ground over the stairway and put a solid door over it, laying on the frame.
The root cellar has a dirt access "stairway", dirt floor and dirt walls, and it works wonderful. We keep potatoes, carrots, turnips, etc. clear into the next summer. You can use pvc pipe for vents.
Someday we hope to have a "real" root cellar with cement walls and either gravel or a cement floor, but meanwhile, we make good use of our free root cellar.
Let us know what you end up doing. I'd sure love to read about other ways people have made or think they could make a root cellar or other cold storage system for root and other crops.