make 'em as you need them. size is determined by what you are clamping
and curvature will depend on what they are made of and how much
pressure you want to distribute.

so here's what you do.

select a stick of wood to make a caul from. it should be a little
longer than the thing you are clamping, wide enough to not dent your
workpiece and a little taller than you need, so that you have
something to take away to get your curve. that piece of wood should
also be a scrap from something else. it's a spiritual thing.

choose a work surface. it should be flat and substantially stronger
than the stick you are making your caul from. get three scraps of
something all the same thickness. put one under each end of the caul
and position a clamp at the center of the caul, clamping it to your
work surface. tighten the clamp the amount that you want to apply to
your work when the caul is in service. now, if you are making a caul
to spread force out from one clamp at the center, rather than from a
clamp at each end towards the center, mark the top, using the work
surface as a reference and something to space the pencil off of it as
a guide. a combination square works well. if your caul will be of the
2 clamp variety, mark the bottom. your third shim block will be the
right size.


now, when you release the caul from the clamp it will have a curved
line on it that is just right for the curved face. use whatever method
you like to shape the stick to that curve, smooth it, round the
corners a bit, give it a coat of wax to keep the glue from sticking
and you're in business.