If we follow dw's suggestion then we have the ultimate safety measure; a/c cannot be hijacked if they do not exist. I realise that due to this (and other similar) incidents that the cockpit will be closed at all times, but unfortunately with terrorists willing/eager to die the only way to combat them is by preemptive strikes / improved security stopping them getting anywhere near the a/c. No current design is capable of simple modification to permanently seal the cockpit - any don't forget the many regional a/c with no solid partitions, just curtains, and even if you had sealed cockpits and Rambo-style sky marshalls on every flight, consider a light twin (or heavy turbo single) loaded with HE or even ANFO - the Oklahoma city bomb was far larger than the WTC or Pentagon impact. It is impossible to protect every possible target.