I have three daughters (4, 6, and 8 years old) that are always drawing pictures for each other or writing each other notes and sliding them under their bedroom doors. So I put these together in Fusion 360 to give them proper mailboxes. I've included the files from Fusion, but I'm a novice so ignore anything weird I may have done. I have included a blank lid in addition to the lid with the letter M on it (added in slicer).

***NOTE***

The keyholes on the back used for mounting to the wall are not sized appropriately (too small), so the box cannot be removed without removing the screws. I had already printed all 3 of them before I thought to measure and haven't gone back to edit the model yet.