Step 3. The letter compartment door
Cut and shape the letter compartment door to suit so that it fits below the top shelf [e] and covers middle shelf [f].
Hinge the door in place by fixing two smallish hinges to the top of the door and top shelf [e].
Any other fixture is personal preference.
If security is not an issue, then just a knob to open the door and some type of catch to keep the door closed or even a spring (as shown in fig.3) to close the door.
If security is an issue, then some type of latch-set or pad bolt that can accommodate a pad lock will need to be added.
Finally, just a bit of paint and the numbers.