I saw an article where a very innovative woodworker made some clamps for his workshop that worked like a cam cleat on a sail boat. ( http://www.garagewoodworks.com/video.php?video=v39 ). They are very useful for holding a work piece on the bench for sanding, filing, or other shaping. A piece of wood is cut with a decreasing radius curve in its outer surface. The theory is that if you place two of them on your workbench and put a work piece between them, the force of the wood against the curved surfaces increases the pressure on the sides of the work piece holding it in place. The outer surface of the clamp has a piece of sandpaper to increase the holding power. The clamp is movable, easy to build, and self adjusting. It works great on the modular side of my work bench.
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.
Step 1: Make Some Rough Blanks
I had a piece of 12 x 12 veneer core plywood in the pile. I cut into four 6x6 pieces for the blanks. Included is a JPG of the curve shape. I have a friend with a laser engraver, so we used that to make the rough shape. You can use the attached JPG and adapt it to the available stock in your pile. Print it and transfer the shape to the wood. Cut one, use it as the template for the others.
The exact shape doesn't matter. I filed off the rough edges with a rasp and file. Then sanded the edges and applied sanding sealer to all surfaces.
Step 2: Finish Them
I sanded the blanks and applied a sealer. This helps to keep glue from sticking if they get spattered. I Filed the edges to remove the band saw marks. Then I stacked them up and sprayed the edge with 3M spray adhesive and attached thin strips of fine sandpaper to provide some extra grip. My workbench has 3/8 inch holes drilled in the surface for bench dogs. These clamps use the same holes.
Mount the clamps as shown so the workpiece fits between the two sides. rotate he clamps to contact the workpiece. Push forward slightly and feel the clamp engage the workpiece. That is all there is to it. You can now hold a workpiece securely to your workbench for sanding, shaping, or sculpting.