I was trying to make something similar to a dress pattern given in Blueprint magazine a couple of years ago. The construction is very simple; a straight tube of fabric, gathered at the neck with a drawstring. I used a white XL t-shirt.
For the Blueprint dress the drawstring was made of soft cotton rope, but since my project will be a nightgown, I wanted the drawstring to have some give, to be soft, & to be plain; no big bows or knots. I dug out a perfect piece of soft cotton knit (vintage!) which matched well. I did have to add some wide eyelet to the bottom; my girl is tall these days.
Here's the link to the original Blueprint article & instructions.