If you want bags, I found the Platypus 2+ liter to be nice. It is a reasonable size and it stands up. That said I find it works just as well to either use recycled sports drink bottles or bottled water bottles. Stick a few in each pannier (or just under the flap). That way you can pick up or discard them and avoid carrying anything when you don't need it.

So maybe carry one Platypus 2+ liter and augment it with recycled bottles or just use recycled bottles exclusively (other than the bottles in your cages).