I may be doing something similar. My basement follows the upstairs profile - including two additions. One of those basement rooms is about 8' X 12', and has both a door & an outside window. It also has a vent from the HVAC which could be used for ventilation. I've long been considering converting that to a cold room, to use for seed storage. The room is dry; but for seed storage, and because there is living space above it, the project would require vapor barrier & drywall on all sides. It would also need some additional work to make it rodent-proof. Since that room is large enough to double as a walk-in cold pantry, maybe it's time to quit "thinking" and start "doing". (where's that 'kicking myself in butt' emoji)