Kuryakyn has some good options that have a large velcro strap that will go around the back rest. I have had the same one for years and seldom use it and it is a discontinued item but it fits the Ryker great. It's kind of a round bag that has openings on the top and sides.
I recently got the backrest and I am thinking I could attach a bag to it. Anyone know of some options?
I've been wanting something similar to mount on the back of the backrest but not rest on the back seat. I may need to fabricate a plate of some sort that has mounting holes on the sides to mount a standard bag using straps.