RJ,
The shower curtain may not be offering much diffusion. Depends on its density.
Also, modifier offer more gradual shadow transitions when they can wrap the light around the subject.
A good rule of thumb is twice the diagonal or diameter. So, at the distance you shot this it would have to be a really big modifier.
You have a nice balance between the ambient and the strobe.
For this type of setting the harder shadow edges are fine in my opinion.
I would have tried to frame the shot so the dark tree branch wasn't pressing down on her head. 