A 2400 x 1200 x 12mm panel will weigh approxim
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"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