The best way to increase airflow through your radiator is to drop the pressure behind it. These louvers from Velox Motorsports are designed specifically for the ND chassis Miata, shaped and positioned to take advantage of the greatest pressure differential available for maximum efficiency.
Powdercoated gloss black to match the factory aero pieces. Bolt-on installation using hidden studs means no exposed cut edges for a clean finish. The louvers come with vinyl templates so you can be sure everything is exactly where it belongs and cutting up your hood is as unscary as possible. Designed and built in the US.
The optional wickerbills improve efficiency. Rain trays are available if you're concerned about rain getting into your engine bay. The underhood components do have some waterproofing, so this isn't needed unless you live in a very rainy location or you want to minimize dust and water spots.
Note: Velox Motorsports has changed their name to Verus Engineering - but the parts are as good as ever.