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.
Here’s a short video showing them being tested.
And one of them in my garden.