I cut the parts out of a mdf board (with a lasercutter) and put them together.

Than I glued the WS2812B LEDs on the inner surface.

..and wired them.

• The black, green and white wires will be the data and power supply for the LEDs in the corners.
• The red wires are vcc and ground. ( I changed them, because I thought they were too thin, but I just took a picture with that wires )
• The one red wire on the top is data in for the first LED.

And finaly I just put all together.