These 4-foot stands are constructed from 3" PVC drain pipe and a couple of blocks of 3/4" MDF. The PVC is cemented to a flange on both ends, which are firmly screwed to the MDF panels. Before I put the top panel on, I filled each pipe with fine silica sand, thumping the pipe up and down on the ground to pack it firmly. They're nice and solid, although 4" PVC would be that much closer to earthquake-proof...