With regard to the "unknown" mast thickness, it's a relative simple
trick to drill a very small hole
(about 7/16 is right ) through one wall of the mast, and then fashion a
small "L" shaped hook.
Put the short part of the "L" through the hold, bring the long part
horizontal, and then put a little
dab of magic marker flush with the mast.
Remove the hook, and measure the distance.
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