Made it as simple as possible. Did take some more thinking and planning, as it is so easy to make complicated solutions. Needed are: tent , Thule WingBar or Aerobar roofrack, plywood base 2150x1250mm, 2.6m telescopic ladder and Car Rooftop Tent DIY Fasteners. One weekend is enough for the project.

Pictures of construction details can be found along with bundle of Rooftop Tent Fasteners.
Set up and packing video is here.