The sheet I used was a little thinner then I would have liked, so after interfacing the outer pieces I went ahead and sewed a piece of white fabric to the back of each (using a 1/4 inch seam allowance). I then sewed the bag together just like normal. This gave an extra bit of stability that it wouldn't have had otherwise.
The bag has a large zipper pocket in the back, two front pockets and an adjustable velcro closure.
Here are some quick pics I took of the bag before packaging it up.
Check out my tutorial if you want to make your own messenger bag!