Abstract: A method of making a fluid cooled assembly that incorporates a base that forms a partial enclosure defining an interior void space and having a top wall that has a top surface and that defines at least one opening through the top wall to the void space, the base further defining fluid entrance and exit ports into the void space, the top wall being made of material that can be friction stir welded. A lid having a size and shape substantially conformal to the opening, having a top surface and a bottom surface that defines a set of downwardly extending pins, and that is formed of a material that can be friction stir welded to the base is placed into the opening so that the lid top surface is flush with the top surface of the base top wall and friction welding the lid to the base.