There are some thimbles of various plastics that are solid center except for the pilot hole for drilling to the exact pin size, like the bronze thimbles for a spliced wire eye. Hard to find. Could work.
I'd not trust any of the open white plastic thimbles I've ever seen. They often have guide loops at each side of the throat as they are often sold for use on an anchor rode where streach in the eye is an issue. I've seen them collaps after serious (but not storm) use on an anchor.
It'll be interesting if any other readers know of improvements in the plastic units. There are some dead-eye bullets for synthetic rigging, but these are more akin to the solid center I mentioned first. Perhaps someone has a good unit.
If you're going with an open thimble, I've had satisfaction from SS, this on staysail schooner mast with a main up around 750 feet and opposing the maybe 300 foot main staysail.
G'luck
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