basically what you need is a wedge.much like airleron or trailing edge stock.you say the widest part of the gap is 1/8".then cut a blank rib out of 1/8 balsa.all you want is the rectangle shape,correct length and width.you will need a piece of 1/8" music wire and a piece of sheet metal about 20-24 gauge would work with one straight edge.sandwich the blank rib between the wire and sheet metal.use tape or clamps to hold hevry thing in place while you sand the balsa sheet to a wedge shape the wire and sheet metal will keep from sanding too much away from where you need the thickness ans will give an uniform edge.after you get the wedge glue it to the outer rib and cut to shape then when you join the outer panel the gap should be gone.