These Porteur bags are made right here in Maryland. The material is ballistic nylon so it's really tough and water resistant..The bottom is padded and there is a pocket at the back. The zippers are waterproof, so there's no need for a flap.
It can be carried with the included shoulder strap or like a hand bag. There are a couple of ways to attach the bag to the rack, with the buckles as shown above, or by using the daisy chains underneath.
They hold about 22-liters or 1340 cubic inches. The base is 31.5cm (12") x 26cm (10.25"). It's about 24cm tall, but can be extended if using the daisy chains to attach it to the rack. It is water resistant, but not 100% waterproof like a dry bag.