I don't think 'JLaw' looks at all like Rebecca, aside from having blonde hair.
Piper Perabo, however, has a relatively good similarity to the version of her adult depiction from the original 'Book 2' graphic novel, if they wanted to go down that road. Not sure about the brunette version from 'Book 1', though... Lizzy Caplan, maybe.
Although, personally, I'd prefer Rebecca just as a cameo, as illustrated above. She deserves to stay out of these events and have her own settled life.