Hi from Spain:
I commonly use two amazing Bikeplanet tail lights even under the daylight.
1.- One attached to the seatpost and one to the left (from behind) seatstay. I think that the seatpost one helps you to be seen not only from the car right behind, but from the ones behind that too through the first car windows.
2.- One attached to each seatstays. It is simetric and cool, but, as safe?
3.- Other options are welcome.