This may not be the very best
way of going about it(at least, for my situation), but what I did when I moved my boys across the country(....and we're talking a 6-day trip here)was just put them in their cups. We were in a driving RV, so I tucked them away in a dark place surrounded by towels. It was a rather rough and long trip, but everyone made it alright.
If you're just in a car for 6 and a half hours(which...at least to me...doesn't seem all too long), why not put them in their cups and put the cups in your cup-holders(If you didn't keep their cups, you can always go to your LFS and ask if they have any spare cups you could have)? Stick the tanks in the back of the car, and just set them up as soon as you get there.