The hole that has always concerned me is the inspection hole at the smallest diameter of the IV fuselage, just ahead of the horizontal stab. Next time your near a IV hold the tip of the horizontal stab between your thumb and index finger and apply 3 pounds of force alternating in an up and down direction. Increase the frequency until you hit the resonant frequency. Continue applying the 3 pounds of force at that frequency and watch the amplitude increase. Try it with and without the cover attached.

If I had a IV I would attach that cover with many screws and keep them tight.

This does not work on the small tail 360, the resonant frequency is too high.