Has anyone got a good setup for mounting a light on aero at extensions, and not having it upsidedown

I was going to buy a Blackbourne saddlepack for my Trek Domaine but it said 'not compatible with carbon seatpost'. Is this just 'covering their ass' - it's just for overnight trips and I won't be carrying a lot of weight?
Otherwise, can anyone reccommend a 'cheap and cheerful' pack that will do?