Why not attach some velcro inside the flap to secure it, if you are not sure about adding a zipped pocket again why not use velcro either at the top of your pocket or add a flap to cover the pocket inside. Allfreesewing have some great bag patterns you can download and also easy to follow tutes on adding zippered inside pickets, post a pic of you bag when you finish, I'm sure it will be beautiful and unique x.