Cut a hatch in the skin as others have done and glue a ply mount to the far side of the original, install the servo as per the instructions to clamp the ply in place, that way the ply mount will not change the position of the output arm and get over the probelm as detailed in the manual for bigger screws for mounting the servo's, did you kit not have the bigger screws supplied? if you are having trouble mounting Futaba servos due to the small hole diamiter in the brass ferrral through the servo mount, use JR ferrals they are bigger and will take the bigger screws supplied.

Mike