I've tried several different methods to mount servos in foam wings. Many people simply install rails in the wings and and put the servo in with the top of the servo sticking out of the wings or tails. While this works well, I prefer to hide my servos completely under hatches...looks much nicer and keeps oil and dirt off your servo.
CLICK HERE to see how I go about putting servos in sheeted foam wings.
I have CNC-cut carbon fiber servo hatch kits available. Email or PM me for details.
-Tomm