I put a cardoid in the box with the gear, have been out looking for scrap foam for the salt flats begged, borrowed or stolen version.
Hey, Max.
I think you'll find a shop rag will suffice, and probably be more easily acquired.
While a cardioid mike does reject sound from the back and sides, an omnidirectional mic is less prone to wind noise.
That said, the mic you're using is actually hemi-directional by design - I short handed it to "omni" just for brevity, and in the application you're using it, it's functioning in the manner of an omni. It's a commercial contractor Crown PZM which was initially designed for use in a wall mount application. Sound is reflected off of the surface of the wall and into the element. By not having the wall as a reflective surface, a lot of bass frequency is lost, but that's not a problem for what we're trying to pick up on the salt.
So if you see someone driving an Omni with a Hemi, just throw it in the glovebox.