When I mentioned to a friend my questions about the best design for a soft shackle, he replied that I should simply duplicate the commercially available design. When I went "Huh? I don't know of anybody selling these" he muttered under his breath about "provincial Americans" and declared that innovations in sailing technology are born in Europe, and pointed me to the online catalog of a
German chandlery. A
close-up shows it in use.
The manufacturer is
Liros (select English as your language and search on Shackle in their product list).
Their design looks to be a sliding eye with a diamond knot.
John D.