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And if you have a carradice type bag, that also often blocks the light if your frame isn't large.
THat is why on my Isen I went with the son mudguard mounted option and have been really happy. Wiring is nice and tidy inside fender with gorilla tape, then into frame at BB.
On my other bike I solved this like this
The SON seatpost mount light is very nice and does a similar job, but is expensive.
You can get a clip from B&M that mounts to the saddle rail and sits on back of saddle.
https://www.sjscycles.co.uk/lighting-spares/busch-muller-rear-light-saddle-mount-adapter-50-mm/
I'm in the process of designing a 3D printed mount for the rear of a Gilles Berthoud saddles (which have two threaded holes on the backplate) which gets it nice and high, and not in the way of a saddle bag if that is on the rails.