Originally Posted by
HiYoSilver
We're not talking about much weight, just bulky: winter gloves, extra base layer, goretex slip on pants.
You don't need a 1200cm^3 bag for carrying that little stuff. One of the larger Jandd seat wedge bags can easily fit that. Check out the Jandd Mountain Wedge III, and go to a fabric store to get some additional velcro to make a seatmast extender to fit around the aero-post.
I've got a 12L Berthoud on my brevet bike. I can fit everything (food included) for a 400k with 30 degree temperature differential in there with room to spare.