I was compelled to return to the Palace Of Animal Death. Only because they have great affordable bike bags, albeit disguised as fishing tackle bags. After deliberating for several weeks, I finally pulled the trigger (pardon my language) on a sweet, waterproof-looking bag that seemed like it would be right at home on the front of the fatbike.
It would mount here, on those eyelets in the black tubing that never seemed to make sense.
A strip of reflective tape across the bottom, and the job is finished.