I currently use this cover and this harness with one of the seatbelt hooks that connects the harness to one of the seat belts for my five month old Newfoundland puppy.

It works pretty well and it's made from a super durable fabric. The only downside is that it doesn't have any sides to protect the doors or stop him from going into the floorboards, my biggest issue is that he likes to slip between the door and the side of the hammock to get to the floorboard.

I have a coworker that has a GTI and the seat hammock that he has for his car does have sides, I'll ask him where he got it and I'll post a link once I find out.