L&M Taz 1200 & Vis 180 Tail Light, clipped to a 'L' Lezyne seat bag...(bike frame 17.5) works!
Of course a smaller bag, or no bag, will allow you to clip the light right to the seat post. I just like to travel prepared, and so use the larger seat bag for tools, etc., and a hydration pack for first aid kit, food, extra clothes, or jacket, as needed, etc. Ya, kind of like campcycling! Better to be prepared as best you can, I believe.
Note the *side lights on both front and back lights; the side lights on the tail light alone a very, very bright...way more than appears in this shot.