When I found myself in need of a few extra bar clamps, I took a tip from the flooring profession, a line of work with practitioners who are constantly using wedges to force together pieces of wood. I screwed a wood block to both ends of a relatively straight 2×6 (see drawing). One block is perpendicular to the edge of the 2×6, the other at a slight angle. Then I cut a wedge to match the angle of the second block, and had an instant bar clamp.
Chris Petersen, Storrs, CT