Create DIY for this, I guessing you are going to need to use 2 shirts, cut the one you'll wear with the side slits. Then cut strips out of Tshirt, sew into the inside of the first shirt at the top then weave and sew to the bottom of shirt.
How to quickly get a t-shirt to fit better. It is perfect for those work t-shirts that you get stuck wearing but are sized for someone twice your size. Fashion and Sewing Tips: RECYCLING OF SHIRTS AND T-SHIRTS - 2
DIY Tee to Cut Out Bandeau Top Tutorial from Bleached Faded here. Really easy tutorial although Id probably sew wear she has used fabric glue. I would also again wear a tank top or something under it so it's more modest.