Nicely made, Sarah. A sloping rear would fit against the rear seat, as others have mentioned. And I would go for a rectangular shape, about the size of a fish box, with handles on either side. A bit of a compromise, I know, as car boots are lots of different shapes.
I woudln't buy one personally, as I have a dog cage in the large boot of my car (Vauxhall Zafira), and I pack things around that, smaller items in a plastic storage box about 400x300mm.
Good luck with the project