If you have some transparent filament, make some of these to add flair to all your small, dark spaces! Includes a reflector to spread the light evenly, like a tea light.

You'll need a common 5V LED, a watch battery, and some brass shim. There's a tiny amount of soldering, but it doesn't need to be good soldering (and you could probably find a way to avoid it).

See the PDF diagram for assembly information.
Note that the pointy bit in the middle is there to reflect the light evenly, so it works best if that part is white.

UPDATE: smogit had the idea of printing the lamp glass as a thin-shelled solid with 0% fill; if you want to do that, use the lampglass_solid STL file. That way, you can use non-transparent plastic and still get a translucent object.