Hi Mark - I think it is a GREAT idea to send a handwritten thank you card. This is an art that sadly has been lost in the age of technology. I still try to write handwritten notes instead of emailing. I think you can say something to the likes of: "Dear ____, Thank you again for taking the time to speak, I enjoyed our conversation. I appreciate you sharing your perspective on both the business and the position, you helped to provide me with a much clearer picture. I feel confident that, after our interview, I would be a great fit for what it is that you're looking for and hope to work with you in the future. Please let me know if there is anything that I wasn't able to answer for you about my background, or if you have any questions at all. I look forward to hearing from you. Mark"

I think you can edit to add in any specifics that you would like, and to tailor it to each interview. I would definitely recommend adding something different per each interviewer. Let me know if you have any questions and good luck!