I had problems with my laser pointer when traveling. I'd toss it into my computer bag, the button would get pressed accidentally, and the batteries would wear out. I was disassembling it for travel when I decided to make this thing. Slip it on, give it a 1/4 twist, and the button will be protected from accidental acivation.

0.2mm

100%

The two pieces assembled with a press fit. You may want to use a drop of glue if your pieces don't fit as tightly.