If I am understanding right, If you haven't already covered the stab you could easily glue on an extra 1/4" piece of balsa to the "short" side the same thickness as the stab and sand it to shape. Actaully, it wouldn't be that much harder to do if you have covered it. Another thought, couldn't you mount the stab shifted over by 1/8" which will make up a total of 1/4" overall. I wouldn't imagine being shifted 1/8" wouldn't be noticable either.