Ortlieb Back-roller Plus

"The Back-roller Plus from Ortlieb is the ideal rear pannier for anyone who doesn't want to compromise when cycling. With a generous volume of 20 liters, it offers enough space for everything you need on your tours. Thanks to the roll-top closure, the contents remain safe and dry, even in the most adverse weather conditions. The PU-coated Cordura fabric ensures additional robustness and durability, while the IP 64 protection class guarantees that your belongings are protected from dust and splashing water."
"Equipped with the innovative Quick-Lock2.2 system, the bag can be easily attached to and removed from your bike. The large reflectors significantly increase your visibility in road traffic, which is particularly advantageous in poor light conditions. The practical carrying strap offers two fastening options so that you can also carry the bag comfortably when you are not on your bike"
"The Back-roller Plus can be perfectly combined with the Rack-Pack in the sizes 24 L and 31 L, which offers you even more storage space on the rear racku00e4. For even more flexibility when packing, there are separately available accessories such as the Pannier Stabilizing Insert, Packing Cube Set, Commuting Insert, and many other useful extensions that make your bike tours even more comfortable"