Another builder in the area kindly donated his wing jig. I added the cross braces. This jig is on wheels so it is portable, somewhat. It does take up most of my building area, but the advantage will be doing both wings at the same time.
Update: I ended up ditching the cross braces. Too cramped to work on that side of the wings with them in place. With both wings clamped on, it is pretty stiff and sturdy (as long as the wing skins are on).