Comment posted to Does anyone know any Celtic symbols for wisdom/knowledge? by eustacia.
Ask the Irish what they would use.
Or, have a look at Ogham, a form of writing using straight lines across another line, and which was largely used on stones. It is now related to the tree alphabet as popularised by Robert Graves (author) and you can pick one or more of the tree symbols for their meaning, time of year or can run a few together to form a name or word.
This may have the advantage of being ‘secret’ to anyone not in the know.
Or, use any of the curvilinear symbols as from the Book of Kells,