The Tardis caddy
This caddy is very small, but it's one of the appealing things about it. Fits under the seat very neatly, but the inside is larger than the outside. In mine I carry a co2 inflator 2 co2 cartridges, spare tube, tire levers, patches and a multi-tool. Admittedly, if you don't enjoy playing tetris, this may not be the caddy for you. Straps very securely to the saddle rails. 2k miles and zero problems.