A saiplane I built long ago... (Marks Models Bobcat) had the servo slip down on its side, the tabs on the servo would slide down two vertical channels (one on each side) in the hardwood rails. A strap acrsoss the top as described above was used as well. However there was no vibration to contend with either which simplifies things a bit. You could however slide the servo with grommets installed down inside the rails which could be widened to accept the grommets. You wouldn't need to screw in the servo then, just the strap across the top.
Mike