Fairly simple design with some minor flaws that I currently don't have time nor will to troubleshoot. Easy to print and assemble, although so far there isn't any general way to fix it to your case or table. You need just a few cables (I use dupont), standard 6x6x5mm button and a LED - you can propably reuse all the old components from your PC.

Flaws

• the button has .2mm difference between pushed and non-pushed state - that is pretty hard to work with, so the switch doesn't always trigger
• cables everywhere, pretty tough to come up with a way to fix it to something

Solution ideas

• Button-less remake of design, just 2 male dupont cables that touch each other
• More rigid switch mechanism
• Moving the button and the LED to the back of the casing to get all the cables out of the way

Feel free to make your own remixes with the above ideas implemented, or your own upgrades.

This design is really really DIY as you can see from the photos… :D