Well, it isn't exactly muslin, just scrap fabric I had lying around:
I added a "dart" on one side as you can see, and it makes the fit better, but it makes folds in the fabric. I think the only solution would be a seam under the bust like this:
On the other hand I don't want it to be tooo bustier-looking (don't get me wrong, I love bustiers, but it's just not what I'm going for...). And I don't want to have too many seams on the bodice. I need to think about how I'm going to fix it. Maybe I don't need that dart and the fabric won't bunch up under the bust once I have the straps?
the straps need to be fixed first, which is why I had them tied around my neck: