Your answer to Victor to attach with screws is humorous but the old tried and true magnetic attachment previously used for CB antennas is far less damaging, requires less effort, and is removable when the car is parked–just a thought
When I need my hands (which is often), I put the stuff on the hood until I open the door and get into the car. Then, I see the stuff on the hood (Drats!), get out, and bring it inside. No more stuff falling off the roof.