Video: Adam Savage, of Mythbusters fame and now at Tested, needed a car seat for his dogs that could go right in the back of his brand new car -- one that would protect the leather interior, but still be thick and sturdy enough not to move around when they get boisterous. He made this, and it works like a charm. Ultimately Adam wanted a seat that wouldn't move around in the car or get pushed out of the way or onto the floor when the dogs moved around, so he came up with this thick, custom plush pad that covers the whole seat, is secured with dowels in the space between the seat and the seat back, and covers the whole seat so the dogs can lay down and walk around without ripping up the leather interior. Plus, it's relatively easy to make, as long as you gather the supplies (muslin, canvas, polyfill and a few other assorted items available at your local craft store) and have access to a sewing machine.
It's not a perfect system of course -- ideally Adam's solution would have protected the seat back as well and, as a number of commenters at Tested have pointed out, it would have been great if the system could have been "buckled in" using the seat belts in the back so it really wouldn't move. Still, it's a great solution, and one you can duplicate at home. Hit play above to see it in action, or check it out at Tested below.