This is a quick and easy solution to have a light source on your key chain.

You will need a CR2032 cell and a 3mm or 5mm white LED

Slightly widen the gap between anode and cathode of the LED so it is bigger than thickness of the CR2032 cell. To turn light on just press the keychain's body.

For the printing you can use any filament, save some very flexible ones. You can even try to print it in resin. PLA and PET-G are probably the best solution because of better part cooling during printing.

FDM printing time with 0.4 mm nozzle, 0.2 mm layer height and 100% infill is about 20 min for 3mm LED version, respectively 27 min for 5 mm LED version.

No supports, no gluing needed