I just got a bag from Bikers Friend. It is a hard bag that fits on the passenger seat forward of the sissybar. It's held on with bungee straps or reusable zip ties. It is not only a very large bag but a riders backrest with lumbar support as well. It's easily removable and carries like a small duffle. It costs a little over $200 and comes in 4 sizes. I have the XL, it carries almost as much as two regular saddle bags combined, including two full coverage helmets. Worth a look
www.bikersfriend.com