Metal detector plus baggage x-ray plus swabbing/wanding for resolution. Develop a puffer-style device that works reliably.

With that combination, we are already an order of magnitude safer in a plane than any other transportation.

Accept that, no matter what, there is still an extremely small chance a terrorist will blow up a particular plane and there will never be anything we can do to remove that tiny risk.

That's good enough for me. Use the billions of $ for real intelligence. Or to fight a real safety threat.