It sucks, but it's the best I can do...

Forget bullet-proof doors between the passengers and flight deck. The only "safe" door is NO door. Totally isolate the flight deck from the passenger compartment. That way, anything powerful enough to rupture the barrier, also breaches the hull. Tough, but efficient.

I see one immediate problem, of course. The flight deck crew needs to be totally self-sufficient - their own toilets, kitchen, etc. They need their own air bridge to board the flight deck, totally separate from ground crew and passengers. These luxuries would take a lot of space and add costs. But I think they would work.

And THEN imagine the situation if a crew member throws a con rod like that Silk Air pilot on flight 185 (I was on the same route the day before - maybe the same plane and pilot), or the Egypt Air flight 990 pilot?

Imagine the terror of being locked in the back of the bus as the pilot dives for the ground.

Not nice, but it makes it much more difficult to repeat Tuesday's horror.

(But not impossible. Nothing is foolproof)