I don't think doing a 'Stealth' kit would be very helpful as it is often more about know "what" to do than "stuff" to do it with.

However, I would try to package up a small backpack, Tupperware container or something container of gear and simple instructions with it and see if someplace in the car could be used to store the small package. It can be out-of-sight or maybe under the seat.

The next step is to actually get the car owner to understand about the package, how to use it and why that is important and why you and they should care.