We have different methods for different cars. Long journeys and holidays, we will be in the Estate car and he will be free in the boot. We haven't put a guard up as he couldn't get between the gaps above the head resets and he lies down as we travel. I'm not comfortable with a crate in the boot as I always think a bad rear collision could trap and injure him even more in the crate.
In my car, a smaller hatchback, half of the back seat comes forward to give him lots of space, there's a huge cushion which brings the boot up to the level of the seats. he is secured in his Copenhagen harness by a short, super strong leather lead which is looped through the solid metal D ring that the seat would clip into in an upright position. He has room to shift around and sit up, but not enough to end up past the back seats.
I feel that the Copenhagen is one of the strongest harnesses I've ever bought, I wouldn't use any of the other types I've got for travelling which have thinner straps, or just webbing.