I got the rear "add-a-room" with my new EV. This is different from the rear
"screen, which simply allows you to have the hatch raised yet have the opening
screened for insect/circulation purposes. From your description, it sounds as if
you are interested in the former.

I set mine up last weekend. It's a canvas rig: you open the hatch, throw the
canvas over (there's a rubberized gasket-like seam that fits where the hatch
attaches to the roof), and attach the sides that are near the car to the rear
bumper for a tight fit. Bungees for this are included. The corners not close to
the car can be staked down for stability. (Stakes are provided.) The extra area
is quite spacious--gives you another several feet of room (essentially the
vertical height of the CLOSED hatch door). The room has a "skylight" if you
will: a see-through panel that lines up with the rear window, so that your
"ceiling" is clear. Plus, they were smart enough to provide a screened
(coverable) window at the rear of the rig so that you can get a nice breeze.

The walls go all the way to the ground and are zippered so that you can have
access to this area from the outside as well as from the interior of the van.

There is no floor.

Further questions, let me know! I think it's a great option (although I'm
getting a different one, since one of my zippers is faulty).

Megan