The clamps are unidirectional. They tighten when you push against them, and release when you pull the workpiece back. So they are effective when you are pushing against the clamps. So you need to orient your work process to the clamps.
There is no magic to these shapes. Ideally it would be a gradually increasing radius curve. Mine is a Bernoulli curve, which is an approximation that I could find a nice clean JPG online. It can be adapted to any size piece of wood, I had a scrap piece of 12 x 12 seven ply spruce laying in the recycle bin. Use what you have and adjust the size of the clamp to fit.
Author's note - A later commenter suggested using string tied around a nail (or small disk) to provide a true increasing radius curve. This is a better solution and one that in no way infringes on any patents filed y the above mentioned video. To see how tis process works, look at a cam cleat on a sailboat. This adaptation has been around for decades and has never been patented, since it is an obvious example of an applied physics principle. I disavow all knowledge of Bernoulli, or his principles, or the application to fill scuba tanks using his principles.