The letter to the editor should summarize what you outline in your review comments. You should outline why you think the manuscripts should be judged the way you recommend based on your comments. I, as an editor, find that it is useful to get a personal view point and often these are more explicit than what the reviews show. That said, remember, however, that your review is one of two or more and that they are an expert's view. Anything you express has to be supported by facts. Emotional outpourings are not very constructive and will likely devalue the review.
So, I do not see a particular structure very important it is more the content. It is important to get a summary and conclusion of the review outlining the major short-comings. It is good to provide a sensible justification for the suggested "verdict" based on the major points. If the work contains serious problems, particularly of an ethical nature, it is necessary to provide a very good case.