A bag that can be used as a back pack is a good idea but zippers = leakage no matter what you do to them. Maybe a small Wolfman (waterproof - bombproof) might be a better bet. They have carry handles that would work as a back pack in a pinch.
Protect that very expensive headlight lens - not available separately so figure $400 - $500 for the whole assembly. I like this from ZTechnik: http://shop.nationalcycle.com/perl/cycle/Z5410
Light weight, inconspicuous, easy on & off if needed and durable. Mine has a few pits on it which means it is doing its job.
Even on pavement it's a good idea to have a bash plate that is large enough to protect that vulnerable oil filter and oil cooler.
If you are in Southern California with money jingling in your pocket, do a ride in to Bill Mayer and get a real seat made. http://billmayer-saddles.com/BMS/
After riding cross country on the OEM board you will really understand the need for this type of luxury.