See 308 Making servo bay cut outs with a Dremel router. Do a few practice runs first. 

See 309 I use a template to lay out the servo bay. This is made of 3/32" ply. It has "cross hairs on it to indicate the centre line of the servo and the centre line of the output shaft

See 310 Locate the position on your stab )or wing) where you want the servo to be.  Draw a line for the centre line of the servo and a line for the centre line of the output shaft. Place your template over this location.

See 311 Cut the balsa wing skin away to expose the foam.

See 312 Use the Dremel tool with a "drill bit" router tool. The depth for the first cut is set for the depth of your servo "rails. Then set the cut deeper to open out the actual servo bay itself. 

See 313 Go to it !

See 314 Better get out the shop vac

See 315 servo bay prepared
Servo rails cut to size. I put a few "dimples in the rails for the adhesive to grab onto. .Notice the filler blocks to go on the ends above the rails, dresses the assembly flush with the surface