This isn't a report and it isn't a repayment. They gave this hour without knowing who'd use it or what would happen in it. Write back what actually happened, in your own words — that's the only ask. Nothing is scored, rated, or measured against what the hour "should" have produced.
A few honest sentences cover it. If it helps to have a shape:
Dear [donor name],
[your letter]
Send it through the contact page and it goes to Kevin directly. Nothing about it is kept, tracked, or filed — it lands once, and that's the whole transaction.