This works for both C and handscrew clamps. Cut a length of wood of the appropriate size, attach a small piece of flat iron to it (we screw them on) that is longer than the woods thickness so it sticks out enough to drill a second hole through it to attach to the wall. For handscrews if we used a 1X3 the one by 1X3 is vertical so the clamps hang with the screws horizontal and the nose facing down, for C clamps horizontal so they attach like a shelf. Large C Clamps may use a board mounted with 2 screws and 2 more to hold it to the wall. For Pony type clamps use a dowel or a piece of stair railing. Cheap, simple to make and can hold 4-8 large handscrews up to 7-10 Small pony or middle size C-Clamps. Pipe and Bar clamps are hung off something that looks like an oversized towel rack.
We have them sorted by size this way so it is easy to grap one.