The Court of Federal Claims Decision on a post-award bid protest for a sole-source award in response to FEMA's request to the Army Corps of Engineers for debris removal following tornados in 2021 provides insights into writing a good J&A.  

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Although there are specific circumstances here, you can pull that J&A rationale from the court's full decision.  You will have to modify it based on your situation, as the details in this case likely won't be present in your situation.  But rational that is clearly thought through including an analysis of why other acquisition methods would not suffice will lead you to a better J&A.

In this post-award bid protest, the plaintiffs, Ceres Environmental Services, Inc. (“Ceres”) and D&J Enterprises, Inc. (“D&J”), challenge the award by the United States Army Corps of Engineers (“Corps”) of a sole-source contract issued without competitive bidding to DRC Emergency Services, LLC (“DRC”) for debris removal in Graves County, Kentucky, following a tornado that struck on December 10, 2021.