Serfas Speed Bags. I've used the small and expandable medium for a few years, no problems with chafing.
In a minimalist bag, the Lezyne Road Caddy is the best I've found for the money at around $20. The wide velcro strap attaches securely to the saddle rails, not to the seat post. The only catch is that if you carry a CO2 kit you'll need a minimalist inflator. Inflators with larger knobs like the Planet Bike Red Zeppelin won't fit alongside a CO2 cartridge, tube and tire lever in the main pouch, and multi-tool in the slip pocket lid. I carry the Red Zep in one of the Serfas Speed Bags, no problem.