I use this shop-made jig to hold boards on edge for planing, sanding or rounding corners. First cut a deep groove in a piece of 2 x 6 scrap, using either a regular blade or a dado cutter, as shown. Make the groove a fraction wider than the stock you are working so, when the jaws are tightened, the jig grips the work tightly. Follow up with a relief cut down the center of the groove. I use it repeatedly until the jig splits, then I simply replace it.
– Bruce Hertig