Ring to keep the M2 from wandering around your work surface, but you can still easily move the M2.I printed mine in PLA. The tolerance on the inner diameter is tight. My M2 is older - I don't know if they updated the feet at all. Outer diameter of the feet on mine is 38mm - I made the ID 39mm on the ring. Adjust the OpenSCAD file as necessary. I'd recommend screwing down one right, place the M2 down with the other ring loose, then screw that one down where it sits.