I like your idea and partly your design. Think that it would be excellent if you use this as starting image and rework it. Text could be more powerful if you use different font/handwriting and emphasize it differently by enlarging some words out of this invisible rectangle. The image in the background could fade into t-shirt color on some edges of the “frame” so it wont look “glued” on canvas. I hope you will find my suggestions helpful and friendly as I wouldn’t write this if I didn’t find your idea very, very interesting and unique.