A friend of mine asked me for an UV light source to check bank notes so I quick draw a two coin cell (CR2032) and a 5mm UV led holder. No switch, each leg touching both sides of cells and some pliers work.

The cap needs some work, I attached his using a few drops of Bondic (I'm seriously addicted to it, it's amazing) as non permanent adhesive.

Keep in mind that you are bringing 6 volts directly to the led, so it won't last for long, but it's easy to replace once it's burnt (and gives a lot of light)