I've been contemplating doing this for the last year. I've decided to finally take the plunge ad make a full carbon unit that models the apr unit.
Ill start by making a perfect plug out of wood/plastic with bondo to smooth everything out, then I will make a fibreglass mold from it, then I will use that to create my 100% carbon custom rear diffuser! Ill do a write up and ill make a thread as it progresses.
The best part is that I will have a great mold that I can use over and over again if needed.