Have you tried the Inside Line Equipment porteur bag? I have them on a handful of bikes that I use for getting groceries, etc, and all of them are fitted to Surly 24-pack racks (which they fit and fasten to perfectly). They do not have internal stiffeners (though I'm sure you could make your own), which is great because they fold down and catch less wind when they are empty or nearly so. Below is a pic of the full size ILE porteur attached to a 24-pack rack on a Riv Bombadil. The bag fastens to the rack with two straps/buckles that run under the length of the rack from front to back. They are beautifully made, expand to a huge volume, are always in stock, and priced very competitively.
Willet M.