I thought you might like it.
I specified stainless steel and aluminum but I agree the corrosive effects are a problem. I wanted cooling air only with the Coanda boat tail, but then there was that discussion of exhaust driven cooling and I succumbed to peer pressure.
I don't know if the airflow available would support that large a duct. Since the boundary layer makes it half-way down the back window unassisted the final duct might not be much larger that the engine compartment lid. Mimicking the Bird of Prey, it might stretch from out to out on the fenders and be no higher that the beltline.