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At less than 5 pounds, the relatively light weight of this lens gives you some options for support. You can get away with a regular ballhead, but for flight shots and panning, a gimbal or sidekick-style support would probably make life easier and perhaps yield better results. If you can give a price range that is more specific than "small budget", it would enable people to give specific recommendations.
As for support in a vehicle, plenty of cheap and easy options. Some pipe insulation cut and laid over your open door window works, as would a bean bag or even a pillow/rolled up jacket. If you're so inclined, there are ways to mount tripod heads on the door frame as well, but given the limited space, I would try to see if the easier, simpler methods work for you first.
As a side note, if you are so far away as to need a spotting scope, then even with 600mm you'll still need to work on getting closer.