When I get fish I'm going to be in the same boat, 3-6 hour drive home before fish can see my tank. I've thought a lot about this. My plan is to build a small travel aquarium out of a cooler that won't slosh around during momentum shifts. It will be insulated, heated, with an air supply, (most likely a small airstone), running off of the AC
jack in my car.
With travel time that long you have to worry about how cold they will get, especially this time of year, and how much oxygen they'll use. It all depends on the needs of the specific fish. Ie.: If the fish you're transporting just ate, it will use more oxygen trying to digest it's food.
I'd like to add a question to the discussion: Could harmful amounts of ammonia build up in that time?