For about six months, I lived out of a 1990 Nissan Sentra 2 door sedan
To create enough space in the rear area (passenger seats and trunk) that I could lie down with my legs outstretched, I remove the rear seat and back rest, as well as a particleboard wall separating the passenger area and the trunk. From a foam shop in Palo Alto, I bought a 5 x 8 sheet of 3 in. polyurethane foam, which I cut to (roughly) fit the trunk and seat area.
I had the rear side windows, and rear window tinted as dark as possible (95% light blocked). A casual observer could not see in, if you didn't have a light on, and they weren't close to the vehicle.
To hide my sleeping self from prying eyes, I bought a blue window curtain from Walmart. I took the curtain to a seamstress. She doubled it up, thereby making it opaque, as well as shortening it enough so that it hung just below the seats. I hung the curtain from a spring tensioned shower rod placed just behind the back seats. The curtain and rod could be removed quickly if I didn't want to make it appear that I was living in the car.