I think the only practicle way of preventing anny more unlawful interferences in the future is to have Skymarshals on every flight. Having armed crew can be more dangerous if things get out of hand and hijackers will always find a way to get into the cockpit, even with a reinforced locked door.
There is a limit on what can be done with the national civil aviation security program and it sounds like this time, the hijackers have just used brute force, only skymarshals can prevent this !!!