Foos, I make a pseudo rib from 1/8th ply and cut a servo opening in it and the real rib, then I glue the pseudo rib on the side of the real one. The ply rib and stock rib are plenty stiff together to stand servos forces. On some I sheet the bottom under the servo and cut a slot for the arm. On others I place a couple of pieces as you did for the slot.