travelling

My 2 are pretty bad at travelling, even short distances They pant and flop within 5 minutes and take about 20 minutes to recover one they are home again. However, I have only ever had them in their carrier, which is big enough for them but not enormous - what I mean is theres no spare room in it though they can flop together.
Now I'm wondering if they might travel better if they were loose in the back (I have an estate car) - although I can see the danger of one making a running jump out of boot once I open the door. :roll:
Or maybe I should get them a big open dog crate for in the car - would that be better?
Any suggestions / ideas? :?

Sounds exactly like Feegal, my other three are not too bad. Personally I would keep them in a carry cage as if they freak out when loose they could take a running jump into the window or something, and really injure themselves. In the summer, I put a damp towel in the carry cage to keep them cool.