Okay I put a couple of hours on this tonight - please excuse the model it's slightly ghetto but it'll do the job. I re-used a couple of parts from other models. I also couldn't quite get the curvatures of the roof right so I've curved the spoiler instead to create the same gap at the edges. There's a higher quality render running right now and I'll post up pic once thats done.
*** Thumbnails are for the clicking ***
The spoiler I've bodged together:
The velocity in the right plane before:
The velocity in the right plane with the spoiler:
