Alright, after talking to W Durable, I'm going to go with them. They can do the bag in time for the trip I'm taking next month, and their prices are extremely reasonable. Moments ago I got a price quote from Watanabe in Japan, and they want three to four times more. Of course, the Watanabe bag would be much fancier and more lavishly crafted, but I'm happy going with the more utilitarian approach.
I need to pick a color for the canvas:
Thinking about Cobalt Blue, Smoke, or Vat OD (bottom of the image, military-spec). The bag will be trimmed in medium or dark brown leather. The strap will be leather, too.
So, I guess the key question is: how far do I want to carry the military-inspired style of the bag?