Bonnerphil,
What I sometimes do is strap a small bag on the front of the factory backrest. Mine, believe it or not, is a black cordura fabric bowling bag made by Brunswick. They're really inexpensive but tough and a heck of a lot less in price than the specially made bags for bikes. It actually looks good and is especially comfortable on longer rides. But of course you can't use it while riding two-up. I slit the pocket on one end of it and that end slides snugly over the padded factory backrest.
Hope this helps.
Dwight
