I think I would put a 1/8 piece of balsa on the end of the spar and sand it to fit the spar, and glue the rib and sand the extra balsa to the rib so it looks like it belongs. Not enough to make a difference, but use a nice piece of balsa to fix the leangth instead of scabbing in a bunch of dust that will look ugly. The balsa will look like it belongs at least.