This are simply the best bags we have found and tested.  Bags are sold individually.  If you need a pair make sure you order two.  Designed to be mounted to our side rack system. 
 
Slide in system for quick mount and release.
Water and Dust Proof inner liner
Reinforced back for added protection
Wide mouth, roll-top closure
Customised heavy duty buckles
Double hooks for added security on the roll-top
Fit most pannier racks system
Carry handle
Mount inline or across the rear seat / rear rack
Seamless welded waterproof liner