There isn't a knot that this as strong as a shackle. My thinking is that if the line is designed to take the load (including the stretch), then a knot's strength is a much smaller fraction of the strength the line.

Getting West Marine, or a rigger, or putting one in yourself isn't a big deal.

I know a knot will work! but if you ask what should you do (not, 'could') -- there is a reason most boats have eyes and shackles.