most of the issue with my home made extenders was that the tolerances weren’t tight enough. When the clamping pressure was applied, they pulled themselves out of parallel, causing a the work piece to want to bow.
Since this, I have purchased quite a few Bessey made clamp extenders, and they still suffer from this issue, albeit to a lesser degree.
What overall length were you doing this Jon? I noticed that too, but it wasn’t so bad for whenever I was using them up to 8 feet or so that I couldn’t still get the clamping pressure and alignment I needed. Mine are all the older style K-Body’s, not the Revos.