DIY Macrame Back Tee Shirt Tutorial from Trash to Couture here. There is a link to a really good video on how to macrame the back that Trash to Couture made. From personal experience I find it better to err on the side of too long of strings/rope/tee mate
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.