I'd want it as accessible as possible from the saddle -- although stopped would be fine, not necessarily accessible while riding. I'd prefer an outside pouch angled toward the rider while seated, although it may be less secure so a good closure and/or flap might be appropriate.

One-handed operation so the zippers and closures would need to be really good and smooth, non-binding.

Outside pouches for quick access might be best with an elastic closure, depending on anticipated usage. A slip pouch might be handy, depending on whether I also used a handlebar bag with map pouch in the top like my old Kirtland Tour Pak.

I really dislike Velcro for anything other than arranging the positions of adjustable dividers. Velcro for temporary/quick attachable-detachable mounts would be okay. It's my pet peeve with camera bags. I won't carry a bag that uses Velcro for closures on main compartments and outer pouches. Besides being noisy, they're difficult to operate one-handed, inevitably trap stray fibers and gunk that causes the closures to become less secure with age, and just generally seem like a cheap and nasty solution that could be better handled with other types of closures. That's one reason I generally prefer Domke camera bags, and look for a similar design in any bag. Billingham and similarly designed (and expensive) classic bags also use closures other than Velcro.

One reason I like my cheap trunk bag - actually a rolling beer cooler/picnic bag - is because it uses good sturdy, smooth zippers for the main compartment and outer pouch. There's also a mesh/net elastic pouch on the outside but I seldom use it.

Sounds like a fun project, looking forward to seeing what you make.