The Sci-con Vortex 480 RL 2.1 is a small bag that can carry lots of gear thanks to smart design.
The keys are the top, which zips completely open, and the cleat that attaches the bag to your seat rails. The first means that everything inside the bag is easy to see and reach. The second secures the bag without compressing it, holds it securely while making it easy to take it off, and hides two tire levers in the design.
The roller 2.1 mounting system consists of two sections: the spring loaded clamping cleat you tighten on to your seat rails, and the mount atop the bag which you use to twist on and lock into place. There are two stops on the mount. You twist, hit the first stop, then lock with the second. The clamp has the added benefit of not abrading rails or shorts or saddle.
Figure you can store a tube, mini multi tool, a few CO2 cartridges, inflator, patch kit. There’s a reflective stripe around the bag for safety and a loop for hanging a blinky light as well.