Not cheap, but I love it, and it fits the into same general aesthetic as the acorn bags. Gilles Bertoud.
Wallingford carries some. They are
here.
The one I think I have is
here.
Here's a photo of my Long Haul Trucker with Nice Racks (front and rear) with the bag I'm describing.
If you're interested, I can find a ruler and measure my bag to determine which model I have for sure. It requires a KLICK fix mounting thing, which some folks don't like, but I found handy.
I rode across the United States last summer with a full set of these bags and the panniers still look new.