The correct form of the surname is Fornůsek (as seen on his social media:
https://x.com/fornusek_martin) and the correct feminine form is Fornůsková (note the dropped 'e' on the diminutive ending).
It is apparently a variant of Fornous which comes from a dialectal or Polish word referring to a type of manorial servant who receives some of their goods as partial payment—the equivalent word in standard Czech being "deputátník". Unfortunately, I couldn't find the original word or the name of the dialect.
My main source is here:
https://www.prijmeni.cz/Fornůsek/puvod (requires a free account to view);
And my source for the meaning of "deputátník" is here:
https://cs.wiktionary.org/wiki/deputátník