Luckily, the curtain's width matches nearly the vehicles inter width.
To fix the curtain, I used a shower curtain rod. Since the inner of the vehicle slightly wedges, I had to come up with something keeping the rod from falling down. My solution: hooks made from a steel coat hanger.
|right side hook|
|left side hook, with curtain|
|the curtain installed ... not pretty, but get's the job done|