I have boxes that are nearly 20 years old and still working great.
I explained the door thing once before on a thread. Essentially, I save the rectangles that I cut out of the inner and outer walls of the box and make the door out of those. The rectangle cut from the outer wall of the box, the part of the box that is exposed to the outside world, is the inside wall of the door. Since this rectangle was cut from the same piece of plywood that makes this wall, it fits perfectly. I make the 2x4s that frame around the door stick out 1/4 inch all around and the door fits flush up against the 2x4s so it can't fall in. I then frame the outside of the door, cut and install the insulation, and use the other rectangle that I previously cut from the inner part of the wall to make the outside of the door that is exposed to the outside elements.
Its very easy to see in person, but it sounds so complicated when I try to put it into words. The cut out from the outer wall of the box becomes the inner part of the door.
