Someone recently asked about my clamp racks that I use for the K-Bodies, UniClamps, Tradesmans, etc. Below is a picture of how they are constructed. Dimensionally, they are 3" deep and 3"-4" tall and made of scrap 3/4" plywood. The slots for the clamps are made by drilling holes repeatedly across the top on the DP, evenly spaced at about 1.25-1.5" and then cutting the sides of the slots using the miter guage on the table saw. (guide lines are drawn to provide a reference for the blade and it's helpful to mark the TS table top for the cutter boundries. The hole size determines the slot width and should be appropriately sized for the particular clamps that you are going to support. While you "could" cut the slots with a dado setup, that would require having a very, very large spinning cutter raised up really high...something I'm not comfortable with...as the slots are probably about 1.75" "deep".
Assemble with glue and brads/nails (I usually use my 15 guage nailer for this) and then mount to the wall with 2" (min length) screws into the studs. Easy, quick and cheap. Look great, too. You can make them as long or short as you want or can fit in the given space. If you do really long, you may want to consider an intermediate support made the same way as the ends are shown.