I love looking at all the t-shirt quilts. Each one is so unique. When I did a t-shirt, I did the bias seam on the outside borders like you have done. My long arm quilter said. "Don't do bias seams on t-shirts because it stretches and you get the wavy borders." I noticed my borders did not lay flat but took it to the long armer anyway. Luckily, it wasn't much, and she was able to quilt the fullness out. Hope you have fun doing the rest of your quilts.