So, in actual use, when you tighten down on the knob, it tends to grip the PCB and pull it along a bit, sliding it along the bottom surface of the PCB.

So I added a component: a washer made from EPDM rubber, cut from a bit of leftover garden pond liner (aka, roofing rubber). 

I added a picture to the gallery to show it.

I cut about a 2cm square of rubber, drilled a 6 mm hole in it, put a bit of all-purpose cement on the bottom "jaw", put the rubber on, and tightened the top nut.  After a few hours to let the cement dry I trimmed it to the circle with a craft ("Exacto") knife.

Much nicer now: the board doesn't walk around when you clamp down on it.