I impress myself and cobble a vertical brace for the rack from two spare support bars that I've been carrying around for ages
Maybe my trip hasn't ended just yet!
I'm not sure that these two bars screwed together will hold the rack in place, but my first impression is that it works. Now, should I continue on to Keystone Canyon some seven miles further, or turn back while the bike is still functional?
The vertical silver bars constitute my repair; the horizontal silver bar attached to the brakes is part of the normal Old Man Mountain rack installation on mountain bikes.
I never thought I'd use these two spare rack-support bars, and certainly not for this unintended purpose.