The cockpit doors can be ten inches thick and made of solid lead. If a terrorist demands "open the cockpit door now or we blow up the plane", doesn't matter how secure the doors are, if they carry through on the threat.

I do however endorse the Sky Marshalls idea but with several not just one on board each flight, and also the flight crew being trained in some method of self defense.

As grotesque as this also might sound, all commercial aircraft should also be equipped with a self destruct mechanism to minimize the chances of what happened on September 11th ever happening again.