The cockpit door seems to be the big factor in all this ,so let's do away with it. Weld a big piece of steel from one side of the aircraft to the other , that would require a cutting tourch to get through ,bit difficult to get one past security. Have a letter box system for passing meals through. Crew to enter and leave via emergency exits. Access to the flight deck impossible from the cabin , ever. Problem solved
Guns no longer required .Anything short of this still leaves the possibility of hijacking open.