I have a set of fat tire fenders on the way and I'm planning on a shield for the down tub as well. I'll figure that out after I get the fenders on the bike.The fairing is super cool, as a kid I would have been so stoked to have something like that.
In my experience a rear fender on the bike, or at least a wide rectangle of plastic attached to the down tube of the tailgunner are a good idea to keep the passenger from eating too much gravel.