How I Designed This

I got a small survival kit for Christmas that included a small key light. I really liked this idea and wanted to make a 3D printable version so I could give it to my friends as a gift and other people could enjoy it because I think this idea is very useful in many situations.

Post-Printing

Due to the small size, depending on the quality of your print and other elements such as the use of rafts or supports, a utility knife may be needed to fine-tune some details.
When assembling, you first need to insert the switch in its place, then insert one leg of the LED (negative electrode) through the hole in the upper part of the body and the groove inside the switch, then cut the leg so that it extends a little beyond the switch. The leg should be slightly bent so that it wants to push the switch to the inside of the button. The second LED leg could also be shortened. It is recommended to have the positive electrode longer than the negative electrode to make it easier to determine the correct orientation of the battery.
The battery should slide between the LED legs so that the "free" leg is pushing on the battery, but the "through the switch" leg should not be in contact with the battery when it is at rest. BEWARE OF THE ORIENTATION OF THE BATTERY!!!
Then the other half of the keychain body is pressed onto it. The light should not be on unless the button is pressed or the switch is turned to the on (back) position. If not, you probably need to bend/cut the LED legs to the correct position/length.

Additional materials

1x 5mm LED diode (for the best brightness is recomended white color)

1x CR2032 3V battery
or
2x CR2016 3V battery = 6V = more brightness