This looks like a fun project to do with the kids, so I have some questions! Hope you don't mind.
You basically had a three layer rectangle with half of the zipper along each long edge, and a square about as big as the final small ends cut out of the middle of the short edges?
Why is there an inner layer between the outer and liner?
How do you (or DO you?) connect the two ends of the zipper strips?
Rough diagram attached.
