This question also came up on another scooter forum that I'm a member of.
As far as I know, there is nothing commercially available. The best idea was for someone to make one, as you suggested, out of a bike trailer. I still have yet to see it done. I'm usually the guy to do that, but I don't see the point. Between a top case, saddle bags and under-seat storage, that's plenty of room for whatever you purchase, short of a load of sheetrock and 2x4s.
