Model Car Garage sells a set of thin etched metal saws that are so small they will cut the scribed line on the door and sometimes leave material. A hot knife is not an option in my opinion. You know what happens to the plastic when you apply heat, it warps. Hobby Link Japan also sells etched metal saws that are slightly thicker then MCG's. Even if you accidentally take too much off when cutting the door out a stirp of .010 Evergreen will fill the void.
Happy Modeling