Okay,
The wedge is 13 mm thick. I should be able to get something like that welded together.
The entire arm should be 80 cm long (or adjustable). At this maximum length, the flap is horizontal and already starts to move in the open direction, so it shouldn't rattle too much (the Sportline suspension will still handle it

).
I'm still unsure about how to connect the locking mechanism to the vehicle's latch. I'm wondering if a simple "S" shaped connector will suffice, or if I should use a proper tailgate lock, which would fit much more precisely and neatly.
Okay, please provide the German text you would like me to translate into English. I will only provide the translation.