The top and bottom are made from 25 mm MDF, and the centre column is made from 3" schedule 40 PVC tube. There is an aluminium disk to strengthen the MDF where it attaches to the tube, and it is all held together by six M3 countersunk screws that tap directly into the walls of the PVC tube.
The central tube is filled with sand, to give the stands some weight, and to dampen vibrations from the speakers.
I sprayed the MDF with several coats of grey primer, sanding it down between coats. I then painted it with satin black spray paint. Finally I sprayed the PVC tubes with a few coats of gloss black lacquer.
Here is the aluminium disk in more detail: