As others have said, a Topeak Aero Wedge saddle bag.
Two tubes, tyre levers, instant patches, multi-tool, CO2 adapter and two cartridges all fit easily inside it - I've got one permanently mounted on each bike with that lot inside, along with a frame mounted mini-pump (two bikes have a Topeak Pocket Rocket, the other a Lezyne Carbon Drive Lite which wasn't cheap but is the finest mini-pump I've ever owned).
I used to have one saddle bag and one pump that I transferred between bikes - except I kept forgetting. I got away with it but it was only a matter of time until I got stranded so I sorted out kit for each bike.