A few times a week I ride my Carlton with a Carradice Nelson on a Bagman Quick-Release. It fits lunch, a lock, repair kit, a shirt, water, maybe some shoes / other odds and ends. I added a leather strap to the bag which makes it easy to pop off the QR rack and sling over my shoulder. Plus it looks pretty nice for a bike bag when I haul it into a meeting. Not super cheap and won't stay dry in a massive storm like Ortliebs. But if you cinch up the nylon liner it is darn weather resistant.