Hi {{first_name}},
I appreciate you letting me know. While I'm disappointed, I respect the decision and I'm glad I got to learn more about [[company]] through the process.
If a similar role opens up down the line, I'd welcome the chance to be considered again. The work your team is doing on [[specific project or area discussed]] is exactly the direction I want to go in my career.
Thanks again for your time and the thoughtful conversations.
[[Your name]]