Rube Goldberg solutions
It always amazes me to see the extremes we go to after the fact.
Air Marshalls, locked cockpits, and reasonable vigilance will reduce the odds of a hijack succeeding to the point that no terrorist would spend his assets on a plot almost certainly doomed to failure when there are other, more promising avenues.
Reminds me of the national flu innoculation that was proposed in the early eighties. They finally admitted that the shots would probably kill more people than the disease.
I am sure there are other simple, low cost ideas that will emerge.