I would explore custom bags further. If the bag maker is not interested in doing bags for you, any decent, well rounded local seamstress or tailor could do it as I sewed all of my bags on a simple basic mechanical 2 stitch household domestic sewing machine. I have done several now for all my folding bikes; past 2 Dahons, present Raleigh Twenty & of course my own C type Brompton. By the way, all bags offered by the major manufacturers, Brompton itself, Dahon, Carradice, and the rest are basically the same....a bag is a bag is a bag as far as style or type is concerned. It is the construction details (how durable & reliable), fabric used (water resistant, also durable & reliable), and fits whatever you need the bag to do, is what sets itself apart from the rest. Besides price (I find the ready-made bags made by the major manufacturers way too expensive for my limited budget), I don't have to worry about soiling the bag as I can always do another one in case of crushed eggs, dripping milk....you get the idea. Or make them in fabrics that are machine washable & dry-able.

Here are some of the bags I already sewed for a Dahon (shown), Brompton (these bags are interchangeable with it), and my Twenty (shown):