Instead of using a laser, why not just try to emulate what a real elevator would use. Create a sensor system that would stop/retract the doors if the door itself hits something. That way you dont need a set of complicated criscrossing lasers. Try to searh for the keywords "Safety Door Edge", I believe that is what they call that technology, you might be able to find something if you research on that.
From what I understand there is a sort of protruding part of the door that when it hits something, it will break light sensor within the door and that would cause the door to retract. Or something similar to that.
Good luck on your project.
