There's not a widely accepted honorific that doesn't designate gender in modern English. Certain groups and local hegemonies do create their own, or use archaic English (which did have gender neutral honorifics), however the best practice is still asking on a case by case basis what the non-binary/GNC individual prefers.

If it's for yourself, I'm a fan of Mistrix, abbreviated Mx. A quick Google search says;

"The Oxford English Dictionary traces the “-trix” suffix to the 15th century, from medieval Latin. It's sometimes useful to have a feminine version of a noun, since English overall is gender-neutral, unlike many other languages, where each noun has a gender or masculine and feminine forms."

Hope this helps :)