Hunger of Hadar has a curious wording:
A 20-foot-radius sphere of blackness and bitter cold appears, centered on a point with range and lasting for the duration.
Compare this to Fireball:
A bright streak flashes from your pointing finger to a point you choose within range then blossoms with a low roar into an explosion of flame. […] The fire spreads around corners.
Magical darkness spreads from a point you choose within range to fill a 15-foot radius sphere for the duration. The darkness spreads around corners.
Total cover rule here states:
A target with total cover can’t be targeted directly by an attack or a spell, although some spells can reach such a target by including it in an area of effect.
What would allow or require Hunger of Hadar to appear in a different shape or size?
Most simply this would be mundane walls, though I guess manufacturing a glass container of desired shape wouldn’t be hard, and then there’re spells like Force Cage and Leomund’s Tiny Hut. These would block most spells, but the HoH spell description is very explicit… a 20-foot radius sphere appears, no buts, no ifs. Is there a rule which would override the HoH description?