Since you're securing it with screws, that alleviates one of my major complaints about bike bags: I want to be able to easily remove the entire bag, and carry it with me, using the securely-attached, comfortable HANDLE!

How about adding an outer pocket for your phone? Something that will close and protect it, but doesn't require rooting around in the main compartment to answer a call, or to get the phone out to capture a crime scene video?

I would want some sort of clasp, inside, to hold keys, so they don't fall to the bottom of the bag.

I hope we'll get to see the final product!