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If you drive a lot for work it is a lot easier to bring a folding table. I can fit one into the back of my 2010 corolla which is a very compact car. You could fit either this or this or the slightly smaller version which is this
I know guys at my company who will go to walmart (when they fly) and just buy a cheap folding table, especially if it is a machine we are commissioning.
For seats, I have used my milwaukee packout with a foam kneeler before, and it beats boxes and crates most of the time.
Other ideas i've had include a tri-leg chair & a folding stadium seat
But again, it is super inconvenient to bring this things with you if you fly more than you drive. The folding camping chairs are pretty cheap and can be bought at any walmart for like $10.00. That one I linked from amazon at least has armrests.