I was going through the same dilema, but finally wound up with a used corbin with the backrest.
I saw a bandit with the holeshot setup, and it looked a lot better in person than the picture of it. At least to me, it looked huge and tractor like in the picture, but in person it's smaller and more tidy. Plus, you have the option of removing it, and putting it on when needed.
The third option I was considering was mounting a small trunk on a luggage rack. I was looking at scooter trunks, just for the smaller size. Many of them have a back cushion attached.
I've searched and found no other solutions. I even bought another stock seat (cheap), and was thinking of mounting an old school strap between the rider and passenger areas.
I have the 2nd gen bandit, and I still can't figure out why they changed the handles from one on each side to the useless grab bar behind the seat.