Large Make-up Bag

So I got "brave" and "winged it" for this make-up bag. I used the same tutorial as last time, but played with the measurements. The boxed corners is what threw me off. At first I did them at 5 inches, but that made the bag look tall and weird. So I adjusted it to 4 inches. And the rectangles were 12x14.

The only thing I have left to figure out is how to make it sturdier. I already used interfacing.