True Demiplane

You create a shadowy door on a flat solid surface that you can see within range. The door is large enough to allow Huge creatures to pass through unhindered. When opened, the door leads to a demiplane that appears to be an room filled with whatever you want to be in there, as long as it is less than 10000 gp in price up to 800 feet in each dimension, made of any material. 

Each time you cast this spell, you can create a new demiplane, or have the shadowy door connect to a demiplane you created with a previous casting of this spell. Additionally, if you know the nature and contents of a demiplane created by a casting of this spell by another creature, you can have the shadowy door connect to its demiplane instead.