I've always thought that
Penny (this spelling only) is a darling nn. But I see it only as a nn.
If you don't like
Penelope, how about:
Parthenope ("par-THEN-uh-pee")
Parthenia ("par-THEE-nia")
PetronillaPhilomena ("fil-uh-MEE-na" or "fil-uh-MAY-na")
DaphneApolloniaStephaniePauline /
PaulinaDelphineSeraphinePatiencePrudenceany combination that leads to the initials "P.N."
Some of these are a bit of a stretch. But personally I've never thought of nns being scientifically drawn from formal names anyway.
Good luck!