Thanks all for the feedback!
I'm just starting a new tank on the Mazda/Miata. I'm not expecting anything major or even noticable, but I can at least drive knowing that they
could be working...
I have a little material left over and will make a simple grill block for the car. There are x2 towing eyes in the front grill of the Mazda (why 2...?) so leaving them uncovered should allow enough air to pass in. I can always 'add holes' so to speak later if needed.
Further more, here in the UK we were blessed/cursed with a car called the FIAT Brava/Bravo. I saw one of these beside me at the lights yesterday and noticed some rather tasty looking wind deflectors on the sills. Might see if I can find something in the scrap-yard...