Mark, apologies for joining the thread late... great input from the team, and really like the end product; just the right size for a lock and a laptop on the daily commute...
...couple of observations if I may:
1) one aspect I liked about the US box was the ability to latch lid without locking it; ideally I guess you'd remove the current barrel lock and add some sort of latching mechanism to the back end (like the ally BMW top box), but that would add too much cost/complexity and may not be enough room? Maybe there is a simpler/cheaper latching method that would work alongside current lock?
2) to make most of the feature that you can easily remove from bike, how about a carry handle? Perhaps a single (folding?) handle on one end or two (fixed?) handles, one on each side, that double as bungee rails?