This would be an additional upgrade that can be added to a tool belt in an anvil. It adds a button to the normal radial menu. When you click it, it opens up another inventory with 20 slots.

Just like the radial menu, if you click on an item it puts it on your hotbar. If you keep holding that item, you can hold shift and right click to quickly swap between all of those items. Stackable items can be stored in those slots.

Example use cases:

-Storing multiple colors of glass, clay, etc for building

-Switching between different types of gates or colored wires from Project Red

-Swapping between food items if Spice of Life is in the pack.