From the viewpoint of airline standard luggage, anyway.
I expect it will be a good deal more straightforward on the train.
The mock-box winds up being 75cm x 75cm by 35cm; probably not any harder to buy than the 71x71x28-ish I was hoping for, but rather bigger. Getting both racks—the rear rack, really, the front rack is pretty easy—and the fenders in there complicates the problem quite a bit. So do, apparently, do disk brakes and dynamo hubs.
S&S Machine "Bicycle Torque Coupler" open; not a view one usually gets.