Hope do a few that might work, e.g. https://www.biketart.com/products/hope-m10-stainless-steel-bolts-washers-pair-domed?variant=40809528492211
Ah! Yes that looks exactly the thing. Thanks! All I was coming up with was standard track nuts.
@AlexD started
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Hope do a few that might work, e.g. https://www.biketart.com/products/hope-m10-stainless-steel-bolts-washers-pair-domed?variant=40809528492211