This is mount / holder for the 12mm wide WS2812B addressable RGB LED strip. You have to remove the top extrusion from the printer and insert the piece into the extrusion. From the front / rear the strip will be nearly invisible.
For the ender 3 you either print the 125 mm version twice or the 250 mm once.
I would suggest a brim to increase adhesion.
For all other applications you can scale the z axis depending on what you need.
I also included the .step and .par file for possible alterations.
The LEDs I used are those (de): https://www.amazon.de/gp/product/B01CDTEJR0
will add link to the thingiverse page i found the cable / wire clips on if i find it again
The author hasn't provided the model origin yet.