This is a simple case for an Arduino Nano V3 with an opening for a 3-pin plug. This makes for a very compact WS2811 light controller that can easily fit in your pocket or under your shirt for some DIY LED clothing. Measurements are 50x23x15mm without the plug installed.
The holes in the top are for the LED status lights. If you want proper light guides, snip lock pins off of four RJ45 plugs and push them in from underneath. As pictured, it works well.
One Arduino Nano can power 100 WS2811 LEDs alone with only USB as the power, except if you want to run them at 255,255,255 color in 255 brightness for longer periods.
Designed in Sketchup and exported to STL. Just run it trough your favorite slicer and you're good to go.
Be careful while pressing the Arduino down into the plastic clips if you print it in PLA! You might have to trim some of your PCB if you have a Chinese clone, they tend to vary a bit in size.