Some of the Superstar 40 wings are joined the same way. Mine was. The 3 pieces that are epoxied together to form the dihedral brace is strong enough if installed correctly. If the wood is severely warped you won't be able to bond them together. This 3 piece laminate is stronger than a single piece of wood. It really isn't hard to make new ones. The LHS and even some home stores (eg.Home Depot) sell hobby woods. Take your kit pieces along with you to match the thickness. This material is a harder wood tha balsa (plywood).

Take one of the kit pieces and trace the outline 3 times on a sheet of flat wood stock. This stuff isn't hard to cut with a fine tooth saw. Extreme precision isn't that critical. Hold the 3 pieces together and test fit them into the slots in each wing half. The fit should be from extremely snug to "almost go in". Apply epoxy (30 minute) generously to the complete flat sides of the 3 pieces that will touch each other to make this "sandwich". Clamp together (if the pieces are flat 2 or 3 clothespins will work). Remove the excess epoxy that squishes out from the edges with some alcohol. Let cure for at least overnight.

Mark a line in the very center of the brace. Test fit each half of the brace into each wing half and sand the edges of the brace a bit at a time until it just slips into the wings with a snug fit. Use patience with this. MAKE SURE THE DIHEDRAL IS POSTIONED THE CORRECT WAY. Test fit both wing halves together and assure there is a pretty good mating of the wings without large gaps.

You may find it easier to make the next 2 steps. Apply epoxy to half of the brace (sides and edges) and insert it into one wing half notch that you have mopped epoxy into to the center line you drew on the brace. Let epoxy cure. Test fit the other wing half again for fit then apply epoxy to all surfaces of the other half of the brace, all inside the wing notch, and the complete surfaces of each wing half root ribs that will mate. You may want to tape them together at a couple of points until cured. Allow plenty of cure time..at least overnight. If you confident with the fit you may want to do this last step all in one. Once the glue is cured it's pretty hard to go back and make changes.

Since I was skeptical like you about the strength of this wing joint, I made a small 1" long strap out of some thin bendable metal, drilled a hole in each end, bent the strap to conform with the dihedral angle, and screwed the strap across the joint a couple of inches from the wing trailing edge where there is solid wood and not sheeting.
Wing has stayed together.