Heavy duty Tritium lantern enclosure. Features a curved keyring area. Designed to hold a 3mmx23mm Tritium vial. Comes in various materials. Insertion area is ever so slightly larger than the tritium vial to allow easy insertion.
Note: Buyer will have to secure the vial in the enclosure through their own methods to ensure it does not move or fall out, although the vial does remain inside the lip of the insertion area, and there is a lip at the end/top of the shaft that the tip of the vial rests in.
So all you would really need to do is insert the vial all the way in firmly, then plug/seal the insertion area securely.