Is that a home made aero mudguard and what's it made out of?
Correx, wedged between the SKS and the frame, double-layered (with the grain in opposite directions) at the edge, and secured with cable ties.
Hairodynamics, for the improvement of, but also provides a useful place to attach a race number.
I was deliberately not molishing an aerodynamic tail fairing for racing purposes (if I wanted to be competitive in the part-faired class, I'd have to do it properly), hence the emphasis on durability rather than aero.