A zipper that is 3 feet long, as though it has been torn off of a fine coat. The silvery metal gleams brightly but does not give off a magical glow. 

 Place this item flat against any surface. The surface itself does not need to be flat, the the entire zipper must be touching a solid surface in order to activate the item. Once you place this item flat against some solid object of space, speak a command word or phrase*. The zipper can now be unzipped and opened into an extra dimensional pocket plane. The opening is 5ft wide and 3ft long. The extra dimensional space has a faint glow inside of it and is an circular shape that is 10 ft high and has a diameter of 20 ft. The zipper can be closed from the inside. Once the zipper is closed, there is only enough air in the space for 4 hours**. The zipper can also be opened from the inside using the same command word. The zipper itself is otherwise indestructible to any physical or spell attack. The zipper can be dispelled if a creature casts the spell Dispel Magic on it at 6th level or higher. In that scenario, all objects or creatures within are expelled, and the zipper is disabled for 24 hours. A creature can leave the space either by using a plane-shift spell or some other ability that allows the creature to travel to another plane. 
Once the zipper is removed from the surface it was attached to, all objects inside are expelled. 

*an individual can change the command word or find out what the command word is by beating an Arcana check decided by the DM. 

** there are 4 hours of air for a single medium sized humanoid. That time decreases by 1/2 for each additional medium-sized creature, by 2/3 for a large creature, and by 1/4 for a small creature. (I.e, having a goblin and a human would reduce the time to 3 hours of air, whereas 3 humans would reduce the time to 1 hour).