The rear of the car with the extension is small...
It's shown in the blue lines. Just enough room for two round tail lights and a license plate.
To make this legal, there will obviously have to tail lights running along the sides of the extension to show the "true width" of the back of the car, for anyone following the car down the road. These same lights have to act as reflectors when the car is parked on the road at night.
So I will be making a trip to the junkyard for lots of tail light assemblies that are flat on the outside, with no curvature.
Jim.