I used a National geographic large backpack for motorsport.
It carried;
Gripped 20d, (replaced by 1dIII)
5d,
300 f2.8
70-200 f2.8
24-105 f4
20-40 f2.8
100 f2.8 macro
Flash.
And all my cards, batteries, + chargers, filters. A bottle or two of water. A laptop and power brick. And monopod...