Swytch has one set up specifically for Brompton and or users of the front mounting bag. They have a new look with a slim battery and 60% off if you do a survey. I think prices are higher now (what isn't). My kits were 500 and they are probably 700 now.
I got this survey email a bit ago- I can follow up with the offering price when they launch.
my controls are directly on the battery, I think the new setup uses a phone app.