I'm not sure what I can add, (but that hasn't stopped me in the past;) )I would think you would want a pretty strong zipper, at least a number 5, maybe a 10. One thing to consider if adding an access zipper to the main body of the pack is a way to reduce stress on the zipper when closing by having compression straps that "pull the pack closed" before you zip it up. These straps could have quick release buckles to open up when you want in. They could also serve to hold everything together in the worst case scenario that the zipper fails. A top lid could be simple like the Granite Gear optional lid for their Vapor Trail packs. You also might use quick release buckles on all 4 corners to make it easy to access the pack from front or back, plus easy to remove to use away from the pack. As long as you are tearing the pack open, is there anything you want sewn inside like a security, or hydration pocket? It is easier to sew a pack when open. Scott