I have a quick release sissy bar from HD and am interested in purchasing a sissy bar bag to go on it. My question is how does the bag mount to the sissy bar? I don't see how it is possible. Thanks.

 

It straps around the pad. Some have a pocket (t-bags) some have velcro straps, some have nylon adjustable straps, and even others use bungie like straps.
Most will end up sitting on either your back seat, your luggage rack, or your fender if you dont have a rack.

 

I have a bag with straps. I took the seat pad off, ran the strap between the pad and medallion plate and then screwed the pad back on. The strap is sandwiched between the upright and pad. That way it doesn't move, you can set the height and the straps don't dig into your riders back