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.
Make sure you use a t shirt with very thin, light material. I used a band shirt because I thought it would be so cute but it ended up being thick and stiff. It definitely was easy though! -TM Cute bathing suit cover up:)