I didn't have enough width in the legs to accommodate the width needed for the hip part of the tank, so I added triangle gussets. I almost didn't, but that extra room made the difference between "eh, good try" to "maybe this will be comfortable to wear."
I didn't have pink thread. My philosophy is, if it has to be different, really make it different. With that in mind, I used turquoise thread and top stitched. I thought you could see it better in the photos, but it isn't showing up on screen.
I used the waistband for the straps, positioning them so that my bra is completely covered. I don't know how this will work for an undershirt, but straps are easy enough to reposition.
I used the leftover leg length to create a bias binding for the top and bottom raw edges. I pulled the top edge tight like you are supposed to, but left the bottom edge as loose as possible.
It isn't as refined as a garment made from scratch, but for a shirt that I intend to mostly wear as an undergarment but still want to be useful if it is needed as outerwear, I think it will serve.