This is a cap to prevent the lantern to turn on when you throw it in a bag or pouch and drain the battery.
It also works as a diffuser, depending on the color you print the cap, but be careful with the temperature.

No supports, 2 walls, 0%infill, 0,2mm height, 0,4mm nozzle, normal PLA, no horizontal expansion, once it cools it fits perfect, but it depends on every printer, material, temp, etc.