I have a Thule ski rack as well as a Thule cargo box. Both suffer from a
poorly designed attachment system which uses knurled knobs with nuts
(essentially wing nuts). They have a tendency to slowly loosen from
vibration. On my last trip, one of the ski rack knobs had completely
unscrewed itself and fell off inside the ski rack.

On the cargo box I cut some wooden rectangles with cutouts for the knobs
which I placed over the knobs so they can't unscrew. Yesterday for the
ski rack I drilled two holes on each side of the four knobs and took
some aluminum tent stakes and formed then into an arc that goes over the
knob and through the holes to prevent the knob from turning.

What I really wanted to find was some stainless metric locking wing
nuts, but those don't seem to be available anywhere in the U.S.. SAE
sizes are readily available.