Thinking further, a 3.5 board Wedgeblock shelter could be hinged to fold flat and put on a car or van roof to form a shelter for light adults / teenagers or cheap storage box..

A 2400 x 1200 x 12mm panel will weigh approximately 20 kg, so that 70kg (3.5 sheets) +2 kg for hinges  / screws . Total 72kg

When erected, with thin end of wedge at front, this is reasonably aerodynamic for short distance driving at lower speeds.

  " Issues to consider are the overall limit of 75kg (including the bars), make sure the load is evenlydistributed"

 www.advanced-driving.co.uk/forum/viewtopic.php?f=2&t=2500

For 2 bigger size adults, a 3rd roof bar or supporting bracket bolted into roof may be need. I guess weight limits are aimed for moving cars, so if it was overloaded while not moving, & weight was evenly spread, it may be ok (this is not advice) thinner play or osb will save weight 6 or 9mm, but may bend too much. Going below 12mm on the base may not be advisable