Yet another simple idea that I either never thought of or had forgotten (same end result) : pinner said: If you clamp them to a carpenters square you not only get a true 90 degree angle you leave your hands free to drill the screw into place.
These blocks are like a spare pair of hands for assembling drawers and many kinds of cabinets. Make them out of plywood with dadoes the width of your stock. Forget the dadoes! 1 big square as the base + 4 small squares