Mark,
I guess such nuts will be available but probably not cheap and will ideally require replacement every time they are removed.
I would rather put my faith in a castellated nut and split pin rather than a nylon insert any day. Therefore, were I in your shoes, I would simply drill the necessary holes, either before fitting (using an old shaft as a pattern), or in situ once the nuts have been correctly tightened.
B……
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This reply was modified 2 years, 1 month ago by Bernie. Reason: Typos