We use this SU-TLD in our GNS draft which is also already in the editor queue: <a href="https://datatracker.ietf.org/doc/draft-schanzen-gns/" rel="nofollow noreferrer">https://datatracker.ietf.org/doc/draft-schanzen-gns/</a>
Many many years ago (late 90s) I used to use ".l" (dot ell), nominally meaning 'local', for this purpose, on the assumption that a legit single letter TLD wasn't happening any time soon, and it was visually distinctive.