I've used a large Carradice bag on centuries and brevets, but it feels a little too big and I'm wondering how much air resistance it adds at higher speeds:
On a recent 600 km brevet I decided to try it without the bag and instead put my stuff into a soft bag with drawstrings that I can also wear like a back pack. I wrapped that into a plastic bag against rain and then attached it to the seat rails and seatpost using rubber bands (from cut up bicycle tubes). I used the same setup again for a 180 km mountain ride with a friend.
This solution was very light weight though unsightly and not that easy to use, as I needed to unpack and repack the complete set every time I needed anything from inside: No side pockets for frequently accessed items. I mostly used it for clothes (wind breaker, leg armers, winter gloves, winter tights, winter jacket). I have a separate handlebar bag for tools, camera, USB battery, spare tube.
The advantages of the DIY bag were: low cost, relatively narrow profile, light weight, flexibility regarding packing volume.
At the same brevet I also saw some seatpost-mounted bags by Ortlieb and by Rixen & Kaul.
These bags are longer than wide, which should make them more aerodynamic than the Carradice. Intuitively I worry about the mechanical load on the mounting clamp on those bags, as they are not supported off the saddle or the seat stays. That seems like a lot of leverage for vibration on a bumpy road. Do such seatpost-clamped saddle bags really last?
What kind of rear bags do you use for brevets?