I bought a small-med Joe Rocket magnetic tank bag that has a mount flap for the magnets. I ran 2 inch utility straps through the flap and around the back seat (which I never use for anything but a bag holder). It looks very clean and I just leave it on the seat, and snap it on whenever I need to carry anything...small.
I got the tailbag, and its great!! The price was right, and it fits over the OEM passenger seat with elastic and 3 velcro straps, so its not going anywhere.
No Problem, Don. I took it to the office today and was able to fit my Acer Aspire One, 8.9in netbook with its case, the bike lock and my hat in there and it stayed put.