It was such a pleasure to have had the opportunity to work with you on this project. The extra time, suggestions, and ideas for product modifications you offered our team was much appreciated. In large part to your input and contributions, we have created a unique, high-quality product that we feel will set the new industry standard. Thank you for your willingness to come in and assist us mid-project. We look forward to working with you again on future ventures.
(YOUR NAME HERE)