Sunset magazine had plans to make your own in the late '50's.
I built one for our '52 Willys station wagon. The top was supported by a bow at each end made from electrical
conduit and held apart by 3 spring loaded poles. There was a large screened opening on the "driver's" side covered by a zippered flap. On the "passenger side" was a larger flap which served as the door and it also had a screened opening. At the gable ends were triangular screened vent openings which had folding rain covers. The mattress was a 3/4 bed size. The plywood box had 6" tall sides which, when the rig was folded, contained the custom canvas tent, and it's assortment of hardware.
It was supported on the car roof by cargo carrier racks which were available at most auto parts stores.
The ladder folded in half lengthwise then unhooking a pair of screen door latches allowed the sides to fold next to each other in a parallelogram fashion.
Between the cavernous Willy's interior and the roof top tent, we had sleeping quarters for the whole family. There was also a fitted canvas cover for the "box" to keep it weather tight when on the road.
It looked quite close to the pictured example except the roof had a ridge which allowed one to sit up comfortably inside.
It worked very well for many years until we no longer had the vehicle. It was sold in the early '80's at a yard sale for $100 to become a play house.
Ann & Kurt