I have 2 different bags like this. The first one I got was a
Dowco Ralley Pack. Nice bag soft sided type deal. Can be mounted either behind you and you can use it for backrest or if have passenger mount on sissy bar. Holds quite a bit if packed right.
Second one I have I use mainly if the wife is going with. Packs easier for her because is actually a rollaboard type suitcase with straps to attach to sissy bar. I have never turned it so it could be used as a backreast but I supose you could. It is a
firstgear product.
Shop around. If you dont have to have it tomorrow, there are some sweet deals off and on on e-bay. I got the Dowco bag for like under $50 shipped. The firstgear bag was used in very good condition and around $75 shipped. Also
Leather up and
Jafrum have good selections if you want to go with aleather or soft sidded set up. I have bought from both and they both have good valuse stuff. Be sure to read the reviews as they are usualy good at discribing shortfalls or plus's