Can't think of a last name? You can always just leave it out.
The two and three name system we have in real life is so you can identify idividuals with the same first name in conversation, records, etc. PS does not allow for more than one player to have the same name, so it is not needed for this. The only reason for a last name is to identify the family you belong to... which is also pointless, as no one is allowed to create the same last name either without a GM change.
When the game allows for out of marrage family ties, then a last name may be needed. So don't stress yourself over a last name, just leave it out.