Paper pattern for sure so you can see where need to go.
I would try that piece in one included with the other piece you did. Due to the shape and amount of shrink needed in the centre it would be more workable in a larger area. Then you won't have to worry about welding thick to thin or vise versa. At the moment you have a piece that is being worked close to the edge, so thinning is needed to gain shape. Your progress is going well so far, I admire your challenge.