From Altura's 'Fusion' range of cycle luggage - a twenty litre pannier, sold in singles. 100% waterproof with welded seams and suitable for untold uses. The fixings are 'Klickfix' - these keep the bag on the bike-rack and have spares.
Thanks to using just the one buckle, the 'Fusion' is quick to take on/off the bike and open/close.