I built about 16 of these RGB bollards in 2011. They used the housing from a $2 solar light from Bunnings (no longer stocked).
The tube is 65mm diameter (O/D) x 180mm high. Even with the plastic spike and top cap installed, there is approx 140mm of usable height in the tube. The spike base has one ~7mm hole and a couple of ~3.5mm holes in it that wiring could be passed though.
The solar cell, LED, battery and electronics are all mounted onto a black plastic disc that locks into the top cover. All items can be removed in seconds with a pair of wire cutters, leaving you with a consistent way to mount your new LED PCB or RGB pixel, etc.
I’ve designed a small PCB (40mm x 40mm) that drops in the recess where the solar cell would normally live. It’s a single sided board that uses all SMD parts.
It’s powered from 12V, uses a WS2801 IC and has 3x 5050 LEDs set 120 degrees apart in a 13mm radius.
The power and data connections can be made through the back of the board as I’ve lined them up with the two existing cutouts in the plastic carrier.