I've made a few of them for customers - I wear out all my own T shirts! - and I have always backed the shirts with lightweight fusible. I do sashing around the shirts, and then an outer border around the outer layer of sashing (so the shirts are individually framed, and then the whole center is framed as well). I quilt each shirt with its own unique pattern and try to incorporate symbols, or elements that fit with the shirt's appearance, like a sports team's logo or mascot, or the motto of a university.
Have fun!
Alison