The problem is that fitz is an abbreviation for filius. Medieval scribes used shorthand and abbreviations for many words (heck, when it takes a whole flock of sheep to make one book, you gotta economize). The 't' of fitz stands for the 'li', with the 'z' standing for 'us'. There is really no similar shorthand for the feminine equivalent (filia). I guess you could abbreviate it as 'fita', but the 'a' in this case is not an abbreviation, and I don't know that this was ever done.

So, I don't think there really is an equivalent. It is pretty unique for an abbreviation to be enunciated as a word and then to come into common usage. That's why this is so unusual.