I came across a pretty easy tutorial involving cardboard and wood. Except that this guy has tools that I don't. But I liked his description of finding the shape:
I didn't start with a big piece of cardboard but that would have been smart. I used produce boxes from Costco that the cat liked to sit in and then abandoned. Using a box knife and a lot of tape I managed to get the basic shape:
I cut this out of mdf using a jig saw. Then fit and trimmed a dozen times to get it right. In other versions of this the center had a tendency to bow so I gave it some support.
Then I glued on a $15 carpet from Walmart:
And spray painted the back, this was due to the wood showing in the gaps with the hatch down. It was good for the parking lot and didn't get any attention that I know of, but I wanted it to look purty.
And in the car. There's still some wood showing so I will go back and paint more.
I saved $200 doing it this way.