I have changed the dihedral on numerous wings and built numerous foam wings, all without a dihedral brace.

If you have a table saw or radial arm saw, cut the wing in two and trim the ends square.
If you have completely cut away the center rib, cut out a rib for each side and glue in place.
Butt glue them together on a flat surface with 5-minute epoxy.
Peel back the covering and cover the center section with 6-ounce/heavy weight fiberglass cloth and 30-minute epoxy.
Replace the covering and fly.