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Craig D
Just a few further thoughts after exercising with my Mate’s sextant.  Do you really need to tighten the shades?  Holding your sextant in the normal position the index shades will be well over-centre either 'in' or 'out' and won’t move.  Holding upside down they might fall 'out' if 'in', but you’re only likely to wish to use a shade whilst shooting the Moon when you could manage perfectly well holding your sextant normally.  Index shades might fall 'in' whilst holding your sextant horizontally with the handle uppermost but not if you hold the handle lowest.
I say this because your greatest risk is shearing the screwed part of the bolt/machine screw while you’re working on the nut, or stripping the nut, and then you’d be really screwed (pardon the pun).
If slack enough, there might be easier solutions such as working a thread of dental floss around the shaft between two of the shades.  DaveP