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[Brion Toss]

Hi,
It is a false assumption that you cannot tighten the knot sufficiently unless you have a machine like John's. If you have 4 good carabiners and a 3: 1 purchase you can easily generate over a half-ton, even if you lose the majority of your purchase to friction. It'll be slow, but it'll be cheap.
I use electric or butane backsplices as a last resort, and this ain't one. If you must leave the ends long, a couple of double Constrictors, pulled very tight should do just fine, at least for a while.
By the way, I've taken to tying the long ends together (with an Ashley bend) and tightening the shackle by pulling the ends and the eye. Much easier to get a tight knot.
Soft shackles are still in their infancy, as far as the greater market is concerned. I believe that proponents should make good, handsome ones, the better to win people over.
Fair leads,
Brion Toss