In regard to enhancing the flow of gasses away from your cars interior, in principle this is easy. On the other hand making it stable at highway speeds
might be harder.
  One common appratus on larger vents is exactly a venturi nozzle
made from 2 beetle hubcaps with about a 3 inch space between them. They
are held apart by 3-4 metal brackets on their perimeter and a hole is cut
in the bottom one for the vent pipe. If wind blows from any direction
between them the throat narrows above the vent pipe, so the air speeds up,
reducing pressure right above the vent outlet,......
 Now a smaller version should be easy but it should probable [certainly!?]
be removed when driving.
Bill