a hardened wall with no access from the cockpit to the passenger cabin would be the only way of effectively securing the controls. Sure the Terrorrists would be able to threaten passengers; may even be able to talk the pilots into diverting their course through such threats; but no matter how many passengers they threaten, they couldn't make the pilots risk everybody's life by crashing into Disneyland or some such by threatening passengers though. Of course the problems with that are that they would need food storage and bathrooms attached to the cockpit, which isn't unreasonable; but the pilots wouldn't be able to get up and move around much on long flights (but that may be restricted anyway - and military pilots (non-cargo) fly under those conditions anyway). They would just need their own entry into the cockpit for ground loading.