As you correctly identified, your Kinghorn surname seems to be locational. From a quick search it would seem to be most commonly associated with Fife District (also commonly referred to as the Kingdom of Fife). You could certainly adopt the Duke of Fife Tartan which is commonly known at the Fife District tartan. There is also a newer Pride of Fife tartan.
You could then continue to search your history without having to worry about a slim connection to a clan.
Here in the USA there is a group called the Scottish District Family Association which is active at many Highland games and Scottish festivals.
President, Clan Buchanan Society International
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