For the snack bag, I just use a fabric shopping bag (one of the ones with straps long enough to go over my shoulder)
Speed is the reason - getting ipads etc. in an out of it and on and off the security belt, easy, fast access to snacks, and it's tucked under my elbow so it's secure. Passports/paperwork I put in one of my coat pockets.
I don't find the open top to be a problem.