I was just thinking that to make it lie nicely, do one main center piece that is the triangle part that is seen. I think that part would go up over the collar bone/under the shirt collar to rest at the shoulders, where a shoulder seam would go. Then two pieces that are mirror images that join that and go around the back to fasten. It seems like that would help it lie nicely. In a way, what you want to make is the basic shape (not the triangle part but the fact that it lies around the neck and over the shoulders) as a shirt, except it fastens in the back rather than the front. So just my thinking but it seems that there would need to be seams at the shoulders to give it the right shape and make it lay nicely under the collar. *Disclaimer* I could be totally wrong about all of this!
