I dont have a tankbag but a tailbag is my personal commuting bag of choice. I have a cortech sport tailbag and used it on my 695 and now i use it on my 1098. It fits a big bulky gateway laptop with power cords and a mouse along my lunch and a pair of jeans or shorts.
http://www.newenoughhp.com/luggage/tail_bags/cortech_by_tour_master/sport_motorcycle_tail_bag.htmlIf your using it on the 695 the bungee cords it comes with will work fine.
If for your 848 u can get a tie down strap that use for say a canoe and thread it through where the bungee cords would go in the bag then form an X on the bottom of the bag. The passenger seat will slide right in and keep it secure i can post pics later on if you are not sure how to do this.
If you can't use a tailbag sorry i am no help