I might look at getting a smaller saddlebag (currently using a Carradice Super C Saddlepack), but 90% of the time I don't need that size.I am fairly certain it needs to be seatpost diameter on account of the moulded rubber pad on the device. Maybe you could Whitley down a 27.2mm section of a mop handle?
I have my fly6 basically resting on the seatpost clamp, and the smaller Ortlieb saddle back under my saddle, and I quite like it. It might be slightly letterboxed, but not much, and I think it looks more discrete. I run it as a continuous backup light, and I'd say it still works fine.
At one point I was imaging a stem attached to the seat post, attached to a bit of broom...
Does it just take up 8cm of seat post?