Compact road or cyclocross under-saddle bag.
All you need. If you need more, you need a different saddle bag. Snugs up tight, compact, discreet: no sagging sack under your ass. Holds an inner tube (up to 700×35), couple tire levers, an energy bar and maybe a small multitool. Lightweight, durable cordura outer. Single strap with 3.5″ velcro loop closure goes over your saddle rails and secures to itself underneath. Strap end extends past velcro for easier removal with cold hands or gloves. Reflective tag faces behind you, and is slightly offset to avoid tire spray. Rubberized patch on top flap to protect against seat clamp chaffing. No fancy hardware, or heavy, will-eventually-crack bolt-on attachments, and especially no zipper that will also eventually break.