This letter certifies that Ms. or Mr. x x x has successfully completed a volunteer work (xx hours per week) at (Place) from September 1, 2018 to September 30, 2018.
This letter certifies that Ms. or Mr. x x x has successfully completed a volunteer work (xx hours per week) at (Place) from September 1, 2018 to September 30, 2018.
などで表現する事は可能です。
またボランティア活動証明書に関しては、
"Proof of volunteer work" "Volunteer certificate letter" "Volunteer hours confirmation"
等で表現可能です。 ご参考になさってくださいね。