I wanted to share this with everybody for the sake of lost fluid caps of the future.
I was adding more brake fluid today and accidentally dropped the cap below the motor. Being as there is a solid floor, it didn't hit the ground but instead ricocheted off the floor presumably never to be seen again.
In a panic, I went to Tinkercad and designed a new cap. After some modifications and good measurements, I created a cap that seals on tight and is designed to fit the threading of the Leaf. No idea if this will fit other vehicles, or even if it will do the job correctly, but hey, $13 I don't have to spend
And here's the final product: https://www.thingiverse.com/thing:1848415/