Send individual notes rather than one group email, and reference what each person actually asked you. Panels compare notes, and identical messages are obvious.
Within 24 hours, while the interviewer still remembers the specifics of your conversation.
Email, almost always. It arrives while the decision is live. A printed letter is a nice supplement for traditional industries, never a replacement.
Send each of them a separate note referencing what they personally asked about. Never send the same text to everyone on a panel.