You create a small pocket dimension in the air that holds items in a vacuum. As an action a creature can interact with the 2 ft. diameter portal and put items in or pull items out, but the portal stays within 3 ft. of you. The dimension can hold up to 20 lb. and has a dimension of a 2 ft. cube.When the portal closes and there is something stuck it will be expelled from it.
You can only have one Pocket Dimension at a time, and not having the spell anymore will not remove or replace your Pocket Dimension.
When you cast this spell using a spell slot of 3rd level or higher, you increase the size of the cube by 1 ft. and the weight by 10 lb for each slot level above 2nd. You must use a spell slot appropriate for the contents of the Pocket Dimension.
This is really cool! Only thing I would change is the school. Working with dimensions/planes of existence is usually a Conjuration thing :)
Why is the duration so short? Duration should be permanent. I'd figure the point of this spell is long-term storage of perishable items, such as recently cooked food, or hazardous items that explode upon contact with air. Also, no love for Artificers? This spell would be super-useful for the Alchemist. Up the level by one if you need to balance it.