This is Rhonda Buss's T-Shirt pattern. You can find it on her blog. She put it on Friday and when I saw it I decided I had to try it. I think it turned out really cute. Yes, I got it done in one day.
She suggests cutting the inserted piece 8". I made mine 10" because I was running out of my main fabric.
I think it is very cleverly done. It is very simple to draw the pattern off of one of your standard patterns. This shows how the piece just goes across the back. Give it a try. It is a good way to use up some left over pieces.