I have been machining non conductive parts from Trex for years. It is fairly solid, and fine for parts where rigid forms are not important. Plus these remind me of Tofu chunks.
milling the .125" slot (with an endmill) through the center connecting the bottom slot with the bored hole from he top
bottom right block the pins are visible from the bottom. these will lead the the control circuit and Arduino clone.
10mm RGB LED, and the socket visible within the bored hole lower
Ardunio clone used to control the RGB LED (12 of which will accent the concrete bar) test circuit show
CAD file for the LED socket
more to follow..... see the concrete bar post...