I have always found the issue with record clamps to be is that they cannot apply pressure across the record evenly. They clamp the inside of the record just fine, but do very little for coupling the edges.
Right. I tried a bunch of clamps and they all had that same problem. One that worked the best was (I think) Basis and it used an o-ring around the spindle and another around the perimeter of the clamp. So after some trial and error I came up with this:
https://systems.audiogon.com/systems/8367 Look close, its hard to tell but the washer around the spindle is carbon fiber. There's a bit of Blue-Tack between it and the bearing. And now a more recent mod not in the photo, there's fo.Q tape with a thin film of TC on top of the washer, and a ring of fo.Q tape around the perimeter of the clamp.
The washer is just thick enough to hold the record up slightly above the platter. The carbon fiber clamp is dished out underneath. When clamped down it presses the record flat onto the platter from the outside edges in. Its enough to hold most records perfectly flat.
I know the records are held perfectly flat all the way across because when the clamp is released after play it takes a second, sometimes a couple seconds, and then the record pops up. So it was effectively vacuum clamped even after the clamp was removed.