For clarification I didn't mean to solely rely on the plywood-and-wedge clamps. However they can be a nice addition. Also, a lot of their pressure depends on several things: How thick is the plywood i,e, does it give in, for the same reason the depth; the angle of the wedges and the method you push the wedges in also matter. Long shallow wedges tightened with a sledgehammer sure do apply a lot of pressure...
Another easy to build clamp can be made out of scrap pieces, carriage bolts and wingnuts. Regular hex bolts and nuts would allow using wrenches which should apply any desired pressure. A torque wrench would even allow for using the same pressure everywhere.