Another option is the magnetic snaps that you can find in multiple packs at Joann's in the purse handle and canvas items isle.
I have not made one with them, however I have a friend who has made MANY for sale and other people. She used the magnets all the time.
If you are going to use a zipper, Nancy Zieman has a segment where she puts zippers in her bags using tape that looks like Grosgrain ribbon. You could possily match the tape to the lining material and sew it on after you turn your bag. I just made my first 2 bow-tuck, and while I'm not interested in carrying them, I think if you measured about an inch or so down and sewed your zipper in, it could work without interfering with your pockets
Happy Quilting
Susan