Did you glue the wing together yet? If you didn't I would suggest fitting the dihedral brace some. And try to get the wing halves together. If you are already glued. Make a balsa wedge or rib and glue it in . Then using heavy fiberglass cloth around 4oz weight and glass the joint to at least the first rib out from the joint on both sides. You can glue it with thinned 30 min epoxie. Rubbing alcohol will do for thinner. Or glue it down with thin CA. You must get a flat surface to build on. Here is a suggestion. Go to a lumber store or a Lowes or whatever. Bet a sheet of 3/4" MDF (medium density fiberboard). Cut the sucker in half down the long way and you get a 2 x8 foot board. Take some deck screws 1 1/4" and screw both directions into the MDF. From top and bottom. Find a set of folding table legs. I got mine at a Cole's hardware store. Mount the legs and find something that you can push pins into. I just screwed some homasote to the top of my bench. You can glue ceiling tiles on or whatever you can find. If the bench is too long just cut it down. The table is semi portable and stable as hell. It will be plenty flat. I hope this helps.

Mark Shuman