Please... get real. The way to prevent this is to stop these murderers getting on the a/c in the first place. Once they are on, whether a hijacker or just a stupid disruptive passenges the problem is compounded.
Do you propose we have a steel door with a time lock and a slot where a plate with some food can be passed through? Forget the slot, just leave us with a pack of rations.
This debate has been going on for years here on PPRuNe and the UK airlines have been slated for having cockpit access to visitors in a controlled way whereas the US airlines have a no cockpit access policy. That policy didn't stop this tragedy and no matter how thick you make that door you will always have someone able to coerce the flight deck crew to open it if they have a hostage and start threatening to harm them.
Up until a few days ago it was never even believed that a hijacker would turn his aircraft into a weapon of mass destruction without some sort of communication. Things will be very different from now on.
The only real way to prevent this ever happening again is to make sure that the intelligent services are in a position to gather enough information to pre-empt such attacks in the first place and make sure that security at ground level in airports is conducted in a much more professional manner.
Unfortunately, we reap the consequences of the never ending cost cutting by bean counters when it comes to services with no tangible return. Considering that many airport security workers are paid less than emloyees at McDonalds and receive the most basic and rudimentary training what can you expect.
SO many people moan about the price of a flight without thinking beyond their own comforts. AIr travel will have to become a more expensive mode of transport in the future if you want to lessen the chances of another atrocity. Sadly, too many people forget these tragedies far too quickly and begin complaining about the costs for what is often an invisible service.
The strengthening of the cockpit door is not the answer. Prevention of letting these murderers on board in the first place is paramount. Unfortunately we are now seeing the authorities in knee-jerk mode and the modification of airspace and the extra vigilance at airports can be associated with the words "bolted", "horse" and "stable door".