First of all, sorry that my answer took so long, the week was a bit stressful and I always need peace and quiet to answer.
With everything, I took a tarp and tried everything: different face cuts, then tried cylinders or even both etc.
I struggle with the form because it is a kind of stretched film that goes up and down. If it were just a cone, it would be easy ;-) I don't know which tools are suitable for that and would solve the problem
Thanks for the tip, I looked there, but that's too special to be able to tinker with something.
the option was my first idea, but after 4 hours and all variants i had rejected it
I've attached two more pictures, maybe you can see it better there. Unfortunately I don't have more pictures
I came across the following video, which or something similar would be an option, unfortunately the video is so fast that you cannot see what the work steps are like
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