A Brompton fits into a TNL XL Base Camp Duffel :-)
The fit is snug for width - but the bike fits with the standard fold i.e. without removing anything.
The bag has compression straps on the sides and I used the backpack straps to pull together end ends; it really does conform to the minimum size of the folded bike.
I think I have succeeded in finding my 'no one would guess it's a bike' bag