Mounting the servo on blocks that're glued to the servo hatch is almost always an adequately strong design.

The blocks need to be better wood than balsa. They need to be adequately glued to the hatch cover. The hatch cover needs to be a decently strong wood or plywood. It needs to be screwed in place with at least 4 screws. The screws need to go into decently strong wood (balsa ain't) or plywood. The surrounding wood that supports the hatch and screw locations needs to be adequate to the task.

The small dimensions of most hatch designs keep the torques and forces from being amplified by leverage. Plywood and harder woods than balsa and woodscrews are certainly up to the task.

It's always sensible to strengthen anything on an ARF that you don't feel is strong enough, but don't add appreciable weight doing it. CA does wonders when soaked into anything.