What type of bag are you interested in? Zipper duffle bag style? Clam shell style? Draw string style? Foot locker style? Single or double chair bag? With or without additional storage? With or without a shoulder strap? With or without tie down rings? Water resistant waxed canvas or DWR (durable water resistant) Cordura nylon or rubberized nylon?
Probably a draw string style (kinda like what this one, and most folding chairs come with), single chair bag (I have two chairs, so I'd probably get two bags), no additional storage, no shoulder strap (although a fabric grab handle in the middle of the bag might be nice for carrying), no tie-down rings, probably lean towards DWR Cordura (I want something that won't get a hole so easily rubbed in it if it is "wiggling" while in or on a trailer off-road). I know some fabrics have a neoprene (or something like that) padding on one side, so the tough fabric on the outside with a little padding on the inside would be nice (would probably also be more expensive).
A 38" long Zipper-style duffle bag would also work just as well (if by duffle you mean instead of a draw-string at one end, there are no openings at the end but just a zipper down the length). In some ways that might even be the better option. I'm not looking for anything fancy- just something to store the chair in, and if I had it strapped on my roof-rack it would keep the chair inside the bag dry. It doesn't have to be form-fitting, but I don't want a lot of extra fabric flapping around. If I went the duffel bag route, I guess some fabric grab handles on one or both ends would be nice.
I guess I could bring the folded-up chair and factory bag to an upholstery shop, and we could pick out some fabric and use the factory bag as a template for size (would need to be a little bit longer though; ARB really skimped on their chair bag with this one - talk about thin nylon and just barely long enough for it to not close all the way). There might be a market for these bags if someone is handy with a sewing machine and has access to the appropriate material. I really like the ARB chairs, but with the plastic arms it doesn't exactly fold up like a skinny tube like the cheap Walmart folding chairs (which I have a few of those).
I know Overlanders are famous for overdoing things sometimes (I resemble that remark). I don't need a bag with hand-rubbed leather accents, or titanium rivets, or anything like that. Of course, who wouldn't want an Expedition Portal patch sewn on their chair bag to spice it up a bit?