From the Wikipedia page. It's kind of amazing that nobody thought to do these things before Cooper.

Aircraft modifications

Due to multiple "copycat" hijackings in 1972, the FAA required that the exterior of all Boeing 727 aircraft be fitted with a spring-loaded device, later dubbed the "Cooper vane", that prevents lowering of the aft airstair during flight. The device consists of a flat blade of aluminum mounted on a pivot, which is spring-loaded to stay out of the way of the door when the craft is at rest, but aerodynamically rotates into position to prevent the door from being opened when the airplane is traveling at flight speeds. Operation of the vane is automatic and cannot be overridden from within the aircraft.

As a direct result of the hijacking, the installation of peepholes was mandated in all cockpit doors; this enables the cockpit crew to observe passengers without opening the cockpit door.