Put a small piece of 1/8 ply behind the balsa, cut it to about 1/4" wide by 1" long and CA it in place under the balsa where the screws go, along the inside edge of the balsa. Then drill the holes for the servo using the servo as a template... drill a 1/16" hole. Put a drop of CA in the drilled hole, blow into the hole to clear the hole of excessive CA. Then when the CA cures, put your servo screws in. Make sure that the grommet that comes with all new servo's is in place with the metal (brass) collar facing the right direction (which is with the bevel end down... the bevel end touches the balsa not the screw head). This allows the screw to bottom out on the brass collar and not on the rubber shock mount. This will hold forever.

DS.