Dear (employee name),
Your suggestion that (quick description of suggestion) has been thoroughly studied and analyzed. We have found it to be an extremely useful and helpful idea.
(Details of implementation of idea and timeline).
We strongly believe that after we successfully implement your suggestion, we will achieve (greater productivity/greater output quality/savings/time savings).
This letter serves as a commendation for your cleverness, imagination, and willingness to help the team. We thank you deeply for your great attitude and your willingness to go the extra mile in helping the company with your ideas.
Congratulations on the implementation of your suggestion. We strongly encourage you to keep the great ideas coming. You have definitely proven yourself to be an asset to the team.