I'm making an
ES-3 (as I'm spraying mine with nitro I need the weather to warm up before I can continue) and my neck also fits very well. There were complaints about the neck fit on previous versions of the
ES-3 kit and many had big gaps, but they seem to have fixed that problem.
Any small gaps should simply fill with glue when you fit it. If the glue line is obvious, stain won't stick to the glue, so you may have to get a very thin artist's paintbrush and run it along the join line with a bit of paint of a suitable colour.