Create Stable Demiplane

You summon a large platinum gate within the spell's range. When you enter this gate, you are transported to a Demiplane with a maximum of 1,000 feet of height, 10 miles of width, and 15 miles of length. The floor can be made out of your choice of Grass (with dirt beneath), stone, or glass. This floor goes down for 10 feet, with a transparent wall beneath it. This wall cannot be destroyed in any way. 5 more of these walls make up the walls and ceiling. An identical portal leads back to the material plane. If this demiplane is dispelled, the demiplane itself remains, and all creatures in it are trapped inside. Another casting of this spell is required to reopen the portal.