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