There is a single-side bag available from DP, or was.
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More pics and details here:
http://ducatimonsterforum.org/index.php?topic=29390.0Givi also makes a T680 frame that holds a bag off the exhaust, and bags to go with it.
Monsterparts has the racks on its site for $69.
I have used the frame, and it works but not great, as you'd expect. Probably quite good for small bags and light loads. But hanging the bag out past that exhaust on straps is not the neatest idea.
I assume this stuff fits the 800, can't confirm.
For carrying a lot of stuff, my suggestion would be a) big tank bag; b) add big seat bag; c) add the single-side bag.