I went with McCalls 7120 as it had a neckline I liked and a dart for some shape.
I made a muslin of the original pattern first and tried it on. I used a sharpie to draw where I thought it should curve out, which was right under the dart on the front. I then laid it out on some cheap non fusible interfacing and drew a curve out. You can see the shape I drew below.
I then redrew the back piece using the front as a guide. I wanted to make the 'wings' long enough so I made them quite long and cut them back to where I wanted it after I tried it on. I hemmed the wings before I hemmed the bottom and I made bias binding for the neck and armholes.
I was originally going to use some beautiful Japanese cotton from Tessuti but it is only 115cm wide and I just couldn't make it work so I used some light denim that was 150cm wide and made a band at the bottom using a few scraps.
It's not often an idea in my head translates to real life so I am excited that this worked. It is the exact shape I wanted. McCalls 7120 is one of those great patterns that you can use as a base for altering into different shapes. I can see myself using it for a few more dresses and tunics.