Thanks for the hint.
But who opens the roof half way gets out of the car to put the towel or cloth in and gets back into the car ?
Must be funny to watch that scene... (while the Boxster stops at a traffic light

)
No kidding away, that might work in CA where you open the roof once a year (spring) and you close it once a year (winter). Here in Europe - especially in Switzerland - you open the roof several times a day...
Markus