@Bobby Mac : I'm most interested in this in terms of the spoiler's impact on airflow.
As it stands, square box shape creates a large area of low pressure which in turn, not only causes drag, but increases the accumulation of rain water/mud/dust etc. on the rear windows and back doors generally.
Your spoiler might just be sufficient to smooth the airflow and reduce the low pressure area. Various Toyotas, Subaru Impreza etc seem to have incorporated a similar idea in their SUV/hatchback designs to varying degrees...
PS: I like the spare tyre cover.... I assume it's a bespoke job?