A clove hitch or any similar knot will never come undone if kept under stress, but may work it's way loose in this application since the strain is on the loops themselves and not on one or the other free end.
This is a fairly permanent installation, I'd be inclined to use the approach to stropping that I use all over my catboat and formerly on my schooners.
Make perhaps two wraps - whatever will fill the pin between the forks of a shackle or toggle - and sieze the ends (generous overlap) under the sprit. Since you'll be allowing a little slack anyway to keep the toggle above the platform, you can do the work above and then rotate the siezing out of sight. I'd do three or four siezings about four diameters long each with stitching under and wracked.
Remember, siezing is strong enough for stays on some traditional rigs. You can make them strong enough for this.
G'luck
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