I use a large "dog tent" from the italian company Dogbag. Keeps my 2 Rhodesian Ridgebacks in the back of my WK1 without giving them any chance to chew on anything. It also allows me to open the trunk without them jumping out.
The dogbag products are mostly available in Europe and are not exactly given, but to me they are the perfect compromise between a solid cage (heavy, cumbersome and expensive) and a simple cushion or blanket (not enough protection for my truck). Also, my local laws require that dogs be contained when driving. A 90lb dog licking the driver's ear at highway speeds is never safe...;-). In case of a head-on accident it would also avoid my dogs becoming projectiles aimed at the back of my neck (the "tent" has clips to secure it to factory tether points in the rear of my truck).
This is one US online retailer I could find by googling for 5 minutes:
There are probably others. The same company also makes blankets and tube-like "tents" designed to fit on the rear seats.