When we began work on our 2016+ Miata washer fluid reservoir, our primary goal was to improve the aesthetics; we designed the tank to look killer, make your engine bay a little cleaner, and we also reduced the volume of the reservoir, saving a few pounds versus stock if you're comparing a full Mishimoto tank to a full stock tank.
One of the major benefits of sharing our R&D process among enthusiast communities is the opportunity to learn even more about what we can do to make the best possible product. Once I revealed our reservoir to the forums, I was quickly made aware that there is an annoying flaw with the stock tank: a leaking cap!