1st let me say, this is no subtitute for a real pet carrier but, in a pinch (as we did during a hasty evac. from Hurricane Rita and there were no kennels left to buy) this works very well. It's also cheap.
I bought large, flat bottomed, rectangular laundry baskets. These baskets have plenty of ventilation on all sides so kitty won't get too hot.
For a top, we cut plywood to cover. You can get a piece of plywood cut to your size at any Home Depot or Lowe's store for a buck or two.
Drill two holes on all four sides of the plywood.
Put litter in the bottom of the basket. It will get kicked out all over the place but it will keep kitty dry in case of an accident. We used cedar shavings bought in bulk.
We put two kitties per basket, tied the tops down with plastic cinches that you use to pull together computer cords. You could also use rope or bent coat hangers just as well. Anything which will go through the holes in the plywood and reach through holes in the laundry basket. Tie it down tight. You'll have to cut these ties off when you get where you are going but that is a minor problem when it comes to getting kitty out safely.
We stacked all the baskets one atop the other in the back of our Honda Element and drove at night when it was cooler and the darkness helped keep everyone calm.
We now have the baskets stacked in a closet and they don't take up much room. The boards are also in the closet, ready to be used again. A little rope and we are ready for the road with a minimum notice.