Inspired by the hashkey, but a more flexible diy opensource solution which could be built at home some key ideas:

• Cap and case 3D-printable (will publish files) on mostly every 3D-Printer
• Small PCB with all electric components including:
  • USB Microcontroller (still not sure which)
  • USB connector - exchange you wire to any color and length 
  • optional RGB LED for some colorful backlight
  • Switch (perhaps a mechanical cherry-style one) 

And here some sketch of it: