I have a Brompton bag, but I tend to use a variety.
Standard Ortlieb pannier on a variation of the Instructables mount:
I have a crash-damaged
[1] Front-Roller bodged onto a Brompton frame in a broadly similar manner. P-clips and zip ties rather than using the Ortlieb hooks, because I have undamaged Front-Rollers for use on other bikes. I also kept the Brompton handle and removed the Ortlieb one.
[1] It's no longer puddle-proof, but fine for Bike Rides.
