Assuming that the trial-fit involves the brace not being glued into either side... if you remove the brace, do the wings fit together ? i.e. is the problem to do with the positioning of the root ribs, or i it that the "socket" for the dihedral brace is misalignd ?

If it's the former, I'd suggest gluing some extra balsa sheet to the root ribs and then sanding it away with a good straight-edge sanding bar util you get the parts to fit; if it's the latter then rework the socket if you can - if you can't, then consider building up a "bent" dihedral brace by laminating it from thin plywood slices that you soak & bend to fit (bend before laminating).