Such a simple idea.
Your doors are held closed by the latch, but they're locked into place with a bracket that slides over a rubber bushing. That makes it easy to align everything, but it means the connection isn't as strong as it could be. In a convertible, the doors provide a surprising amount of structural rigidity if they're properly anchored.
These replace the soft rubber door bushings with stiff Delrin. The door is solidly locked into place, allowing it to reinforce the chassis. The effect is of a stiffer car with fewer squeaks.
The second generation design fits the NC and ND more easily than the original. Shipping now.
Installation is quick and easy - simply remove the two bolts that hold the factory rubber in place and replace with this stiff piece. It will have to be properly aligned, and some light sanding might be required for the perfect fit. We've installed them in every car in our fleet, and most bolted right up. Available for all Miatas.
Sold in pairs, enough for one car.