You probably didn't need any help figuring out what we call this area, but here it is. The tail area is padded just like the seat, therefore you can use any material in this position just like the seating panels.
On sportbikes, it is popular to choose a color that mimics the bike's paint job. This will give the illusion of a cowl while still providing for passengers. Please keep in mind that a vinyl or leather material is not going to match your paint job, but can be chosen to coordinate.