"She is making her own costume."
Translation:Elle fait son propre costume.
I'm pretty sure that "son costume propre" would mean his/her clean costume/suit.
My old French prof once gave us a sentence like, "Je voudrais une propre chambre propre," meaning, "I would like my own, clean room." I might not have the sentence exactly right, but I definitely remember the meaning of the word "propre" changing depending on its placement.