What kind of bike? What is the purpose of the rack? (touring, commuting, grocery shopping) Do a search, this has been answered
- P-clamps
- Seat-post clamp with rack mounts (e.g. https://www.amazon.com/Axiom-Trekk-Mount-Seatpost-Clamp/dp/B00TJ6AJLQ/)
Wheel skewer / brake mount racks, e.g. https://www.amazon.com/Axiom-Streamliner-Road-Cycle-Black/dp/B0025UCXEO/ or https://www.amazon.com/Axiom-Streamliner-Disc-Cycle-Black/dp/B004094HY2 or https://www.amazon.com/Blackburn-Cen.../dp/B00GCB4PMA