Here you go. I have deleted some content to protect privacy but you can insert appropriate names/words/phrases/sentences. Like eb2pharmacy said, you can add more to this based on your expertise area and your contribution.
For whomsoever it may concern:
I am writing this letter to attest that Dr. C**** W**** is a reviewer for P*** R*** L***, an international journal devoted to all areas of general physics. Dr. W was invited to review manuscripts submitted to this journal in the area of high energy particle physics because of his substantial expertise and recognition in that area. As a referee for P*** R*** L***, Dr. W has reviewed 12 manuscripts till date since his first invitation to review in December 2006. Dr. W's reviews and critical comments on a manuscript were always crucial towards reaching a final decision towards its publication in our journal.
P*** R*** L*** is the world's foremost physics letters journal, providing rapid publication of short reports of significant fundamental research in all fields of physics. International in scope, the journal provides its diverse readership with weekly coverage of major advances in physics and cross disciplinary developments. P*** R*** L*** publishes approximately four thousand articles per year and its impact factor and immediacy index are 7.072 and 1.676 respectively. It is the policy of the journal to select its reviewers based on their expertise and international recognition in one or many of the subject areas devoted to general physics (including statistical and quantum mechanics, quantum information, etc.), gravitation and astrophysics; elementary particles and fields; nuclear physics; atomic, molecular, and optical physics; nonlinear dynamics, fluid dynamics, classical optics; plasma and beam physics; condensed matter; and soft-matter, biological, and interdisciplinary physics.
Dr. W continues to be a referee for P*** R*** L*** and his contribution to this peer-reviewing process is both vital and necessary. We appreciate Dr. W's effort in reviewing manuscripts for P*** R*** L*** and greatly value his assistance as a reviewer for P*** R*** L***. Should you have any questions, please contact my office at the address indicated below.
Sincerely,
Editor
P*** R*** L***