According to JC the area is effectively a 200 x 200 x 30 cuboid because the shape doesn’t change like guards and wards does, which follows the rules for “point of origins of spells” on page 204 of the PHB.
The point of origin of a square or cuboid is any one point on the face of the cuboid (or edge of a square). You don’t need to touch all 20,000 feet or draw an outline or anything anymore than you need to do so for thunderwave, the spell is only strictly cast at the last moment.
If you think you cast this spell from the other side of a dungeon door in an attempt to destroy all extra planars and evil creatures inside, you must remember that it is also true to say that unless a spell states otherwise, an area of effect DOES NOT extend past full cover from the point of origin of said effect when it is cast so you can’t make the interior of a building Forbiddence from the outside and bar the doors for a “burn down the house strats”
Trouble is, this makes making the entire interior of a large building or dungeon to have the spell permanently very expensive as it takes 1000gp PER ROOM instead of for the whole area. But then again the components are only consumed after day 30 and you might be generous to say that they can put two identical forbiddances next to each other and make them both permanent as long as it’s all within the 200×200 cuboid.
It’s a tough spell with a lot of implications and small changes or specific rulings can destroy dungeons or make it way too expensive.