I hope you're well.
I'm just writing to say thanks for your help on the project last month. We really appreciate you stepping in to support us. I know you had to put in extra hours to catch up on your own work, so thank you very much.
Your expertise was vital for this project and we couldn't have finished it successfully without you. You really helped us and we hope to work with you again on future projects.
I'll be in touch again soon about possible future collaborations. I look forward to working with you more closely this year.
All the best,