Can't you make sort of a reverse flag that falls down below the box when the postman opens the door to put mail in the box? That's what one of my neighbors has done.

Our mail sometimes comes as early as 2pm, and sometimes as late as 8 pm. It used to be you could set your watch by our mail lady, 3 pm +/- 15 minutes.. But after she got transferred to another route, the times went to 5pm +/- 3 hours. No pattern to the variations that we can figure out. And, calls to the Post Office have fallen on deaf ears.