Custom Key Signature in different clefs

I want to have a custom key signature for a variety of instruments in both treble and bass clef. When I drop the custom key signature onto my score, it doesn't transpose into bass clef. But when I create a custom key signature for bass clef and try to drop it onto the Cello line of my score, it changes ALL key signatures, bass and treble. How do I create a custom key signature that reflects the same series of flats correctly for treble and bass clef?