One does not need to have more t-shirt fabric to make borders and binding. In fact, after making a few of these quilts, I find 100% cotton works best for those outside pieces. It is not a problem with the fused backing on the t-shirt blocks. Here is a link of mine which was posted some time ago. It may give you an idea of how borders can change up the view. Good luck. Yours is looking very nice. I like to corner stones inside the quilt.
http://www.quiltingboard.com/t-90121-1.htm