This is an equilateral triangle Polypanel that holds a WS2812 pre-soldered "pixel". It can be combined with other Polypanels to create light-up 3D shapes.
You'll want to use a translucent filament for this. I've had good results with 3D Solutech's clear PETG. PETG is good since it's tough and a little bit flexible.
This is a two-part design - the "top" snaps into the bottom to hold the LED. If printing this in PETG (recommended), use the "led-holder-top-tight.stl" for the top. It has slightly bigger tabs, since PETG is flexible. For PLA, use "led-holder-top.stl".