I have owned, and broken, two clip-style seat bags so far so I decided to try one with a simple strap attachment instead. So far this bag has survived half a season and a handful of races with flying colors. The strap can be a little difficult to feed through your seat rails but once on feels secure, even without a secondary strap to attach it to the seatpost.
This bag also holds more than I expected. Though a tad tight, it holds a 29" tube, CO2 cartridge with inflator, a multi-tool, SRAM powerlinks, and a car key.