I have this clamp, though haven't used it in a while so I'm going off of memory here, but why does your proposed disk need to be able to freely spin? I recall placing one of those round adhesive rubber disks on mine to soften the impact on the underside of whatever I'm clamping to. In your situation, would it work to drill the bottom out of a rubber stopper and epoxy it over top of the part that's protruding from the threads?