Here is how I would do it,

make a 1/8" Plywood plate that will fit between two ribs with a hole cut out for the servo

Glue it between the ribs about 3/16" below the surface and reinforce it with Triangle stock.

Using 1/4" stock, build a box around the servo cutout - About 1/8" larger than the cutout on all sides.

Sand the tops of the box even with the ribs (Be careful not to sand the ribs - taping some tin foil to the tops of them is a good way to keep from saning into them)