I had bought a piece of elastic to make my own bungee for about £3, but on receiving the 2015 Cali, we found that there are reinforced strips which force the roof to fold inwards. HOWEVER, this doesn't work that well when the wind blows in the wrong direction, as once it turned inside out and could have made things worse rather than better probably.
I still haven't fitted the bungee as if any damage is done through stretch marks, rubbing, or otherwise, this won't be covered on the warranty. It will work if you want to press the button and do no checking though, but you need to make sure that the bungee is intact each time. One day it will perish knowing how toast falls...
But as others have said, putting your head up through the hatch gives you a good sense of things going well. You can pull in the fabric from there at the front if you need to and the back is unlikely to get trapped as it's not flappy enough. If the wind is howling though, make sure you open the windows on the leeward side.