If I'm not mistaken, you're talking about the bridging between the leading and trailing edges. In my construction efforts, I've found its easiest to simply cut the balsa slightly oversized and then use a sanding bar to take off the last little bit. A few quick strokes with sand paper makes small adjustments possible. If you overdo it slightly, you can fill the gap with microballoons. Much of the structure's strength comes in compression when you put the covering on or sheet it (depending on the plans).

Brad