Quite some time ago I had Morris make me up a clamping caul
"template". Basically it's a 48" long piece of Masonite with a curve
cut on one edge to a radius of 2000.5 (inches, I believe). The goal
was to have the ends be 1/8" higher than the center when place on a
flat surface. Well just recently I got around to making the actual
cauls themselves. I had a bunch of baltic birch rippings left over,
so I laminated three pieces together and template routed the curve
into one edge. The finished dimensions are 1.5 x ~3 x 40". I made
them 40" because that maximized usage of material, and because I
needed them for a 36" round table top. I ended up making six of them,
and they really worked like a charm. I used them in pairs, top and
bottom, but I suppose if you had a really rigid bench you could just
use one on top to push it flat. In that case, though, maybe you'd
want them a bit wider - say 4" or so.

And, for those who think it ridiculous to go through the trouble of
obtaining a CNC'd template to make a clamping caul.....you're probably
right! To me a fair curve seems like it should apply more even
pressure across the width of the panel, so that's why I wanted it.

Thanks, Morris!

JP