Don't know how popular these are in the States, but in Europe they sell quite well - If you look at the red Vespa, I'm talking about this sort of thing.
I've been looking for one for my DTX360* for a while now, but owing to the price (Shad/Givi ask around £60-80) I've not, as you guys like to say, pulled the trigger as yet. Then the other day I spotted an Oxford fabric cabin luggage bag which looked the right sort of size and would certainly double as carry on luggage if we ever fly again! So for a few coppers over £10 I decided to splash out. Turns out it fits perfectly in my 37L top box with the added benefit of doubling as a tunnel bag whenever I need the extra room.
* It's quite a narrow gap so standard bags wouldn't fit. Would use instead of a rucksack for groceries over spill.