This 90 degree accessory is the classic donkey's ear designed as a simple accessory to clamp to your 90 degree shooting board.The fence runs up one side and down the other providing structural support for the jig and a 90 degree reference edge on two sides.
I drilled shallow holes in 3/8-in.-thick blocks of softwood. Then I dropped in dabs of epoxy and inserted rare earth magnets. My magnets were 1/2 in. diameter and 1/8 in. thick. Make sure the magnet is flush or slightly below the pad surface