I've been experimenting with this on my Kia Sedona, pending getting a Landy later this year.
Best idea seems to be an old window blind of the sort that rolls up with a spring. Get one at a car boot for a couple of quid, cut it to the right width, and probably replace the fabric with something of a similar hue to the upholstery.
I am thinking that the basic idea is to obscure the view of all your valuable in the back, so definitely nothing eye catching!