These feet or for 1/2" threaded rod and nut on my flexy rack. Allows about 3-4mm of adjustment. It's a tight fit for the rod into the center hole by design. I was a little worried about the weight crushing these but, they seem to be holding up just fine. This was printed using cheap ebay PLA.
Tinkercad link : https://www.tinkercad.com/things/hDISTuPeegN