Yet another improvement on the design. In this new design, the shackle is secure on the sheets even before attaching it to the clew. This was not the case with the ring hitch which led to some problems on the water. The ring hitch version may have some good applications, it just wasn't ideal for what I was trying to do. That led me to the design that has the knot in the center but with the eye inside of the knot as shown below. I am using a double fisherman's knot as the center knot. The double fisherman's knot has a lot of test data on this type of line from search and rescue applications which gives additional confidence in the design. It is more stable than a diamond knot in this application in my tests (the diamond knot distorts under load).
Note that the tails of the fisherman's knot are buried in the shackle. This helps to adjust the eye size and also makes the finished shackle a bit cleaner but I consider it optional.
I tested it on my setup with a diamond stopper knot made from the next line size up and the failure was at the eye. That is as good as can be done so that is a pleasant result.
I have written up how to make it along with some more pictures on my site:
Double Soft Shackle
Allen