Anyway, when I saw these target' shirts on the clearance rack , I bought several. I bought a 4x so I could have plenty of width for the old hipsters...and they are the same price.
I cut right under the arms straight across the body and set that aside. It then cut several large concentric circles out of the other shirt. I just sewed them on the hemmed edge in two layers.
Depending on the length you need, you could possibly just turn the upper edge into a casing. I added another same size piece of fabric to make additional length for me. i inserted the elastic and sewed that together. I also sewed it down along the sides of the skirt casing. (this is so the elastic only gathers in the back section.)