Have you thought of using "tube" as the spout?
Also, if you make the teapot first then anneal it and let it cool, you can take an accurate measurement of the opening to work the lid. I would create a LARGE Maria to start the lid, which you can measure fairly accurately -even cut a circle of cardboard that you can hold it close to to gauge your diameter size. It would probably be good to use a steel bowl (check Big Lots or, depending where you live, an Asian grocery for small Steel Bowls or dipping bowls in sets) to slightly dome the maria/lid over a bit if you want to give it a little curve.
You can then add a small stringer "rim" on the underside of the maria; punty it and pull off the rod, leaving a small round atop as the lid handle.
At least if I were daring to give it a go, that's how I'd try it...