I've got my sprites packed into a texture, and I've got their coordinates. How do I turn that into an NGUI atlas?

I need to create an NGUI atlas at run time, and I think I'm close:

 // Create the texture and pack the sprites into it.
 var testTexture = new Texture2D(1, 1);
 var coordinates = testTexture.PackTextures(textures.ToArray(), 0);

 // Get the atlas game object (just a GameObject that I added to the scene with a UIAtlas attached to it) and set its texture to the packed texture.
 var atlas = GameObject.Find("MyAtlas").GetComponent<UIAtlas>();
 atlas.spriteMaterial.mainTexture = testTexture;

 // Add a sprite object with the coordinates from PackTextures and att it to the atlas's spriteList.
 for(int i = 0; i < coordinates.Count; i++)
 {
     var coordinate = coordinates[i];

     var sprite = new UIAtlas.Sprite();
     sprite.name = names[i];
     sprite.inner = coordinate;
     sprite.outer = coordinate;
     atlas.spriteList.Add(sprite);

     atlas.MarkAsDirty();
 }

This almost works. The atlas object does have a sprite list with the correct names, but the sprites are either blank or they have odd rectangles instead of their actual images. The dimensions are also either 0 or 1, and I'm thinking it's because the coordinates that I get back from PackTextures are between 0.0 and 1.0. Is there some step that I need to take to multiply the atlas's size?

What do I need to do to properly create my atlas?