From the plan on Outerzone it's clear that the center section is built flat using a dihedral brace that is flat in the center. Center gets sheeted over, which is good for holding up to rubber bands (use lots of them!). Looks like pretty typical model construction. If unclear, improvise! Main thing is strength across the flat section extending into the wings. The dihedral brace should do that. It's just a model airplane designed by a guy who was a probably a teenager at the time. Make it up...and do a build thread. Good luck to you guys.
https://outerzone.co.uk/plan_details.asp?ID=5645
Jim