I used an old shoebox, and it is the right size for his Happyland figures. I cut the lid off so it was removable and made a door in each side.
We painted it pink (Harry's choice) and I used cream card to make the shaped bit at the front. The windows are light blue card. For the distinctive Volkswagen camper van logo I printed one out that I found on Google Images. For the wheels I used our Rolobox Wheel Kit For Boxes (affiliate link) which are brilliant, we've used them to make lots of different things. They are simple plastic wheels which screw into the cardboard, they also come with a pull cord. Otherwise you could just make the wheels out of cardboard.
Unfortunately the lid for the roof didn't really work as it was a hinged lid. A box with a removable lid would have worked better. I stuck the windows to the roof so that when it is in place you can see the windows, otherwise the box is too shallow for them to fit.
I (sorry, we) had a lot of fun decorating the inside. I used little cardboard boxes to make a bed and a sofa, then we stuck a photograph of a television onto a small box and made a fruit bowl and table with bottle lids. We cut out pictures and a mirror from a catalogue to decorate the inside walls.
We've also used these fab Rolobox wheels to make a double decker bus. If you have a toddler obsessed with campervans you can find lots of campervan crafts, activities and resources over on my other blog!