I thought I'd answer my own question in case someone else has a similar one. I ultimately decided on the North Face Medium Base Camp Duffel, as providing a combination of maximal volume (71 liters) within the cargo door dimensions, sturdiness, water-resistance, and lockability. It held up well during my trip, and I was generally happy with it as long as I didn't have to carry it very far. I couldn't wear it as a backpack, though, and would have liked a conventional shoulder strap option.