I recently treated myself to a new Brompton and was looking for a small seat bag to carry flat-fixing tools, another layer, etc. I was lusting after an Acorn but despaired of waiting months for the opportunity to win their speed-ordering contest.
Then, last night, I remembered an old Swedish Army surplus gas mask bag I'd bought on impulse a couple of years ago because it was kinda cool. I removed the straps, repositioned two strap mounting rings, treated the leather, and mounted it with two leather straps from an old horse headstall from the attic.
(Sorry, didn't realize the first picture was so blurry.)
Anyway, it's just what the doctor orderedóright sized, doesn't interfere with the fold, and the leather perfectly matches the Brooks B17.