Not quite answering your question, but offering the solution that I use. My upper blocks are hung with a homemade variation of a soft shackle made with 3/16" dyneema. Can't speak to it's longevity, but so far it's performed well in a variety of wind over about 10 days on the water. And it's easy to install and remove. Plus the line only cost $2 and I spliced it with my coat hanger fid in about 10 minutes.
Here's a picture:
And a video of how it works. (It's much faster with the use of two hands.)
This spring I upgraded to a the low profile 7:1 Harken setup from an old set of big blocks, which gave me a lot more space, but also was going to add to the length of the sheet I needed by a couple feet. By hanging the upper blocks down the length of the soft shackle (10") I figured I could I save about 7' of line on the main sheet, which keeps the tramp a little cleaner and at about $1.50/ft saves a few dollars. I also avoided the expense of a $25 snap shackle. We'll see if I'm still using it at the end of the summer.
By the way, I have a snap shackle on the bottom block. Don't remember the brand, but it's made in France. Probably 20 years old and works just fine.