You might have something already in the vehicle that could be used. Some jacks have a metal base that you could use, or perhaps a hubcap, if it's metal. (We made a small fire in a hubcap for tailgating "back in the day".) On an older car, the metal cover that goes over the air filter mounted atop the carb would be a nice (although possibly dirty) base.

Of course, you could bring a dedicated item for this purpose, but, just thought you might want to consider things on your vehicle that would do the trick rather than extra kit. Think about how you'd improvise before you need to...