What I would do, if space allows is to make a backplate of 1/8" ply that you can reinforce the backside of the balsa side cutout. On the inside of that backplate I would mount two 3/8" hardwood rails for the servo mounting screws. Epoxy this structure to the inside of the servo cutout hole. Simply putting 1/8" ply for holding the servo screws is not enough IMO.

Vince