I've played with a fairly aero tail fairing on a similar bike (with OSS though)

In my experience, the improvement you can expect from the tail fairing, assuming that you've done it right is only in the region of a couple of km/hr extra in the 30km/hr cruise zone, and drastically less as your speed decreases. (drag is roughly proportional to the square of speed)
The general consensus seems to be that to work properly to get that extra couple of km/hr, the fairing needs to line up with your body... the test being that viewed from the front, an observer shouldn't be able to see the fairing behind you, and viewed from the rear, an observer shouldn't see any part of your body outside the fairing. If you look at my pic above you can see that I've sealed the fairing to my body shape using soft expansion joint foam. (the grey bit), and have included a shoulder bulge and even so I still have some issues with my upper arms and elbows creating drag thanks to the way the steering is set up.
If you have under seat steering with your shoulders and arms blocking the airflow and creating massive turbulent flow, the fairing is going to be of minimal use except as a box to keep gear in.
Other design factors include assuring that the bottom of the fairing is enclosed/sealed as closely to the wheel and other hardware as possible. If it's left mainly open, you're hauling an open drag bucket.