I have a Kreiga R25 backpack (actually two, they were a wedding present!) that I wear on the Z if I am going to the store. I know the limitations and dangers to back packs so there is that. Actually lost a friend who was carrying a helmet in a backpack who went down so be aware of what you carry.
You could always buy another stock seat when they get cheaper (especially after a few get totaled/parted out on ebay)and physically bolt a small cooler to the back of the seat. I did that on my SV 1000 with the rear seat. Ghetto but works and was cheap. I think all in I was around 50 bucks. I will see if I can dig up an old photo.
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