T-shirts will stretch out of shape unless you stabilize them. When I make t-shirt quilts I use a light iron on stabilizer that I get at Joann's by the bolt. It is usually 20" wide. It is light weight enough not to make the t-shirts stiff, but keep their shape. You might try getting a sample of this stabilizer and iron it on a scrap of t-shirt to see if you like the feel of it. I still used warm & natural between the t-shirts and the backing of the quilt.