I made this camper conversion kit for my Citroen Berlingo a couple of years ago. I had looked at lots of videos by people who had done similar projects, taking what I thought were the best parts adapting and adding details of my own.
I first modeled the whole design in card and foam-board to get the dimensions and work out details. Then it was a matter of making the whole thing out of plywood. A friend helped by making the covering for the bad cushions. The design is made so that it can be taken in and out of the car by just one person. The units are not fixed into the car with screws or bolts as I didn't want to damage the car with a view to selling it on later. I used recycled parts where I could, for example the wide unit lid props are made from an metal chair leg.