In list form:
Epoxo-lite from SIG. Used by many flyers for years. Two part, dark brown. Kind of gloppy and heavy by my lights. I find it hard to sand. Dope lifts off it kind of easily, bubbling the finish.
Aero-poxy Lite, out of the homebuilt airplane business. Two part. Pretty easy to use. Sands well. Available from Windy Urtnowski or Brodak or Aircraft Spruce. My favorite.
Balsa triangle stock. Soaked and wrapped. Glue and sand. Lightest. Not hard to do with a little practice.
Leather. Bill Sawyer sells this material in a variety of sizes. From the pattern making industry. Cheap, light, easy to sand. Glue it down with Ambroid glue.
Home brew epoxy-mix 30 minute epoxy and microballoons to a think paste. Smooth on with a round tool. A little harder to sand and probably a little heavy.

Just a note-in general it's better to fill the wing-fuse gap with as much balsa as possible. Glue is heavy. Little chips and shims as needed. Not all kit parts fit all that well. And if the wing construction is off a little. There are some good links to info about these subjects. I'll see what I can find for you.

Aha, here's a good one:
http://www.clstunt.com/htdocs/dcforu...ID1/13537.html