My great-great grandmother's maiden name

I've blogged before about my difficulty in determining great-great grandmother, Rose Newmark's, maiden name.

I now have several more documents than I had before. No spelling is duplicated. Here's the list so far:

Daughter, Kate's birth certificate (informant - husband, Samuel): Sankad
Daughter, Cecile's birth certificate (informant - husband, Samuel): Sonka
Son, Israel David's birth certificate (informant - self): Sandgart
Her death certificate (informant - son, Barney): Sundberg

I think I am still leaning towards Cantkert as the most likely Polish name.