For our purposes, the flag could be mounted on the upper right quadrant of the mailbox post. This keeps it out of the way yet readily visable. It would be weighted and dangle from a spar we would add to the existing post. Thus high winds would not rip it off and weights would tend to keep it hanging down.
We could put a dangle sign below the mailbox that has our name and hours. Unfortunately, it would not be easily read by a passing car on the roadway. However, it would be useful for those who pull onto the driveway. Alternatively, a larger version could be put on the gate.
A custom mailbox that looks like a house or painted in day-glow radical colors would be easily recognizable. However, it would be a ready target for vandals or thieves. Still, it would be easy to say, "Pull into the driveway with the cookoo clock mailbox."
A light-weight, chalice cut-out could be fitted to the vertical post above the mailbox. This symbol would be easily recognized from the road. Better yet, the "flame" could be a reflective, red, wind turbine to add motion. It would be readily recognizable from the road and have significance for other UUs.