As many already pointed out, what really makes sense is a combination of reinforced and/or redesigned cockpit doors AND an enormously sophisticated passenger tracking and "extraction" system on the ground.
What do I mean by a reinforced door? Something that will withstand kicks and bullets. Why not get them specially made with the lightest strong material available, lined with kev on the inside to keep bullets away. This would probably have to extend the entire wall of the cockpit. And the sliding door idea might actually work provided they arent too tensile/flexible. Add to it a CC camera to let the flying crew kow who's knocking on the other side and that should be a substantial safeguard and deterrent against intruders.
Thoughts??