I have a combo pack made by " The SAC " that has a small round & a larger rectangular bag that can attach to each other or separate that I am pretty happy with. They are made of a waterproof canvas & they attach to the sissy bar. I wasn't sure if those are along the lines of something you are looking for, but I thought it might help. I leave the small round attached to the sissy bar, which is attached to the rear seat, and it makes it pretty easy transition for the wife to have storage when I am set up to ride two up.
The small rectangular bag expands like some suitcases do if you need more room and it came with the round bag and rain covers and all the straps you could need to attach it however you'd like. I currently keep the small round one on my bike at all times - plenty of room for a Thomas Guide, a few bottles of water or whatever to drink, snacks, ice packs, etc. and it looks good too. Good quality and only paid $99.