Ever lose a valve stem cap?
'nuff said. ;-)
Print. Remove from print bed. Walk out to car. Screw on the cap. :-p
There are no internal threads. The first time you use it the stem will cut the threads into the cap.
...so be careful how you align the cap with the stem the first time! ;-)
I found my wife's car and mine had two different stem diameters. You may have to slightly adjust the internal diameter to make it best fit your car. There's a variable, "intdia", and a comment inside the OpenSCAD file for this.