All I did was to pick the rib bay that was central to the aileron as the servo bay. This was an upgrade of a kit that was originally built with a single servo. I added two "half" ribs made from 3/32" balsa sheeting. Icreated servo mounts from a laminate of 1/8"ply (no lite ply here) and 1/4" balsa stock. The mounts are as long as needed to fit between the two ribs. Rectangular cutouts to locate the ends were cut from the rib "doublers". Some scrap balsa sticks were fit around the perimeter of the servo mount kinda like cap strips to support covering.

HTH

Oh yea - drill holes in the centers of each rib between the servo mount and the root rib for the aileron servo wire. Iused a 3/8" wood bit.