I have also flirted with the idea of getting a record flattener. Upon trying various clamps, I realized that the Basis Audio reflex clamp will simply flatten any dished record if you use an O-ring that has a larger cross-section.
According to my measurements, the O-ring the Basis clamp came with has a cross-section of 1.7mm. I use a 2.5mm O-ring between the platter and the mat. The inner diameter is 6mm. The larger cross-section is also to compensate for the recessed center area of the Boston Audio Mat I currently use.
The downside of using a reflex clamp is that I need to turn off the turntable to change sides.
Regards,
iSanchez