ORIGINAL: acerc
On the wing joiner's I laminated 1/4ply to replace the kit's. Even if it had not been recomended by those on here I would have done it. That balsa piece is just silly for a plane like this.
Another tip for you: Replace the 1/8" wood dowels that are inserted into the "I" struts with carbon fiber rods for better strength.
When I built my Ultimate, I used the provided balsa joiner because I didn't really expect any wing failure due to the "box type" structure of the finished wing set. I used the "brown" Gorilla glue also and have not had any problems after many, many flights.