Have to agree with point raised by Danny.
These guys once allowed on an aircraft will get to the cockpit one way or another.
If you lock the door, at some point you'll need to have access for food/drinks, stretch or the call of nature. In this day and age when many aircraft are downsizing, supplying the pilots with their own restroom will surely not be worn by airliners operating regional jets (just as dangerous as the bigger aircraft in the wrong hands!).
If the door is locked during flight it would increase the time period to ingress the cockpit BUT for the sake of maybe 1 minute. Pilots would still probably be unaware of the situation. Whose to say that they wouldn't take a cabin attendant hostage and convince them with said shaving kit to open the door or communicate to flight deck that they are bringing drinks in.
Why stop there? They could easily spike both flight crew drinks, then when they're unconcious volenteer their own services as (MS Flight Sim?) qualified pilots.
So the answer 1st lies in detecting these b****rds and keeping them away from aircraft. Anyone caught planning or conspiring to such acts should be dealt with in the most SEVERE way possible. They have no regard for they're lives anyway, so why not give them a helping hand on they're way to they're maker!