We have tried various options with Oscar. I have an estate so space isn't an issue. However the mud all over the back seats/doors/carpets etc was! This was my problem with the harness on the back seat. I tried lots of covers and blankets, but with leather seats, everything used to slip and slide around!
Luckily I have a metal 'ring' attached to the floor of the boot and I have attached his puppy lead to this and clip this to his collar to stop him making a quick escape when I open the boot! Just make sure it is long enough for his feet to reach the ground if he does jump out as he nearly throttled himself once (and nearly gave me a heart attack in the process). I also bought two IKEA fleece throws (2.50 each!) which I have found much better than old towels as they dry really quickly and mean I don't need to clean out the boot so often.
