One of the most disappointing interior features on NA and NB Miatas is the droopy eyeball vents. It’s possible to make them less droopy with some felt or tape, but the best solution is to replace the vents themselves with a better design. This is my favorite kind of mod, where a part from completely different car happens to work perfectly (see fluidic nozzles).
The vents are Mitsubishi part #MR791367. They can be found on the 2000-2006 Eclipse, the 2001-2002 Chrysler Sebring, and the 1999-2002 Dodge Durango.
The difficult part of this project is removing the old eyeball vents without breaking them. The credit card twisting method works best. Slide a credit card behind the vent and rotate it to depress the plastic tabs and easily pop the vent out. Do not try the string yanking method, that will just break the old plastic.
The replacement Eclipse vents just need a trim for length and some sanding to fit them into the dashboard. The center tombstone holes are not the exact same diameter as the ones near the door because the dashboard material is more flexible than the plastic tombstone. It’s possible to make them fit tightly or loose and rotatable by sanding the diameter. I do not have photos of the fitment steps, but they are well-documented in some other guides like this one.
March 2020 Update:
These Eclipse vents are still going strong! I just learned that another option is Jeep Commander vents. Apparently they are plug-and-play, not requiring any plastic trimming, and they rotate freely in the Miata vent holes. They might be more gray colored than black, not sure if the match Mazda OEM interior plastic as well as the Mitsubishi vents.