You'd also recess the plywood plate far enough so that you can add a layer of balsa to sand even with the contour of the wing's lower surface. You'd leave just enough exposed for the retract unit.

Some modelers would just lay in a 3" x 3" piece of 1/4" ply. The large area of the plate also kept the gears from pulling out. Just cut away enough for the gear unit and the leg. Was heavy, though, but many models of the time were a bit heavy. Many Pattern planes of the time weighed 7-1/2 to 8-1/2 pounds. The Mach I was considered "ideal" at about 8 pounds. The mass helped hold speed in the vertical maneuvers.