Have a look at this option - see images on the link below
I use the same mount on a carroof. The mount can be extended up to 3x60cm. The 3 magnets are stronghold and need some effort to remove. I imagine you attach the camera on the back bonnet of the car while the soft roof is down.
Happy cruzing.
…Oh i just read that u need to open the trunks. hope you find a solution.