As the son of the second-cheapest guy in the world, I find it painful to spend hard-earned cash on specialty tools without pondering the possibilities of making them myself. At $12 for a four-pack (plus $15 for the pliers), spring miter clamps offer a great opportunity for a cheapskate replacement. My variation, the cut-ring PVC clamp, works as well as the expensive version.
Start by cutting 2-in.-dia. schedule-40 PVC pipe into 1-1/4-in.-long rings. Slice open the ring, and drill opposing 3/32-in. holes, 1/2 in. deep, into the edges of the sliced section, as shown. Next, cut 3/4 in. off the tips of two 8d stainless-steel ring-shank siding nails, and insert the tips into the holes with a pair of pliers. (I use ring-shank nails, which will stay put when releasing the clamp.)
To use the miter clamp, spread the ring open with your fingers, and position it so that the nail tips grip the opposing mitered pieces, thus clamping the mitered joint. You can vary the width and diameter of the rings and the position of the nail tips to suit many situations.
Ben King, Natick, MA