I'm building a new printer using 2020 extrusion. I needed a set of legs that not only looked good but also added strength to the frame. Plus they had to be adjustable.
The adjustable foot has a 1/4-20 X 1" hex bolt (an M6 would probably fit) that has been permanently held in place by melting the edges with a soldering iron so it would not fall out. There is also a 1" diameter anti-skid pad that I got from Home Depot to keep if from sliding around.