Ortlieb, the most respected name in waterproof cycle luggage, has created a bag for use exclusively with the Brompton front luggage system. The O Bag features the finish and functionality one expects from Ortlieb and will be offered in a variety of colors. Capacity 20 litres; dimensions 500w x 300h x 150d; comes supplied with a shoulder strap, but there’s no need for a rainproof cover.
I have been using this bag personally (in standard pannier mount form) for the last 7 years commuting 25 miles every day. It has held up really well and continues to be extremely waterproof and functional. A little cleaning with soap and water from time to time is all it takes to keep it looking great.