[Facts] Re: Feminine Equivalent of 'Fitz'
I did come across this recently when looking up something relatively unrelated:
(Talking about the history of Irish surnames)
"At first the surname was formed by prefixing Mac to the father's Christian name or O to that of a grandfather or earlier ancestor, Fitz was from the French fils meaning son. A girl added Ni before her father's name, while her mother prefixed Ban."
So going just off that information, there is no feminine version of Fitz since it's used in Irish taken from French.