I wanted a bottle mount on my Brompton for longer trips without a bag. Problem is every cage and mounting location would interfere with the folded size. Well, I came up with this solution...Vincero Design magnetic bottle mount and bottle.
The mount has a magnet where the logo is.
The bottle is made for the mount and has a magnet pressed into the bottle.
This is the view from the side...it doesn't stick out at all.