Hmmm.. Evoque in Germany rare, then the Convertible even more rare! Makes you very unique!
I can only guess there must be a panel you can remove somewhere in the boot to get access to the top fixings!, If you've seen them built the complete drivetrain, Hubs, brakes and suspension get raised as a unit into the chassis and the top fixing bolts done up by hand inside and I doubt the Convertible is any different.
It'll just be how much do you remove to get access!