Adding 1/2 inch extra length to each wing tip
Cut the wing tips off so you are back to a flat rib, put the required thickness of Divinycell foam on the tip of that rib with 5 minute epoxy and micro balloons, sand to the desired rib shape, then round the tips as needed to get the final shape and cover with whatever you covered the plane with. Noone but you will ever know it was done.