Use the roof box directly instead of as a form. Cut the driver position into the back, gusset it appropriately and have 6-8 fasteners to remove it.
You could use a roof box as a boat tail, as well.
Downforce cycle fenders was a joke.
Edit: the nose is better IMHO. Think about a U-shaped clear plastic hatch that hinges open for ingess/egress but fits closely round the helmet just below the eye-slot. If you didn't have the helmet it would knock you teeth out, but the helmet is required. Just make the plastic edge weaker than the helmet.