Thanks, Matt. Not perfect, but good enough!
Speaking just of looks, I still think I'd prefer something that followed the wheel arch line, with "fairing" bodywork fore and aft of the skirt on the bumper and quarter panel.
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Quick road test cam results: the leading edge is solid, tested up to about 125 km/h (about 78 mph). I haven't driven that fast in ages - but have to simlulate coasting down a long, steep hill into a headwind
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The lower trailing edge however was starting to lift away a few cm from the car at that speed. Not surprising, really - it has the shape of a wing. Tighter rubber band on the trailing edge, coming right up...