Civia's Loft Pannier Rack is a great solution for bikes without rack mounts. This lightweight aluminum rack attaches to seatposts from 25.4-31.8mm in diameter with a 4-bolt faceplate. It's a great way to add 20 pounds of carrying capacity and pannier compatibility to your ride.
Maximum recommended load: 20 pounds
Fits 25.4-31.8mm seatposts