If you've added a tour pak (trunk) to your bike chances are pretty good there's an empty space between the rear of your pillion pad and the trunk itself. Why not make use of the space to carry extra batteries, a camera, first aid kit, pen and paper, small spray bottle of windshield cleaner and cloth, trail mix ---- whatever!
I checked out several motorcycle stores in my area looking for a bag that was 10 inches wide, 5 inches high and about 2 inches thick to fill that space.
The Harley stores had one that fit perfectly but it was $89. Not gonna happen.
A couple of online MC luggage stores had windshield bags that looked like they might work but they wanted about $30 plus ten bucks shopping. Hmmm, closer.
Then, while admiring my lovely bride one day the solution came to me. I went shopping at Wal-Mart.
Check it out: Soft leather with a zippered top, a little zippered storage area inside, magnetic snap closures on leather pockets front and sides. Remove a strap, add a strip of Velcro on the back of the bag and on the aluminum trim of the trunk and you're done!
This might also work as a windshield bag.