Use foam blocks and liquid polyurethane two part foam to foam the top of the flap and sand the shape to match the ribs. Use a sanding block at least the length between any two given ribs so that the sanding block s always resting on a rib as you sand the flap to final shape. The .75" trailing edge foam is removed right down to the carbon fiber bottom skin so that when the top is glassed the trailing edge is galss to glass. Micro the foam and glass with two layers of Carbon fiber BID.