Update:
I stopped by Stanford today, told the guy what I was looking to do, showed him one of the brackets off the Honda, showed him the new light (it arrived yesterday) and he went over to a bin and pulled out three stainless steel self-drilling screws. Dropped them in my hand, and told me to come back and show him how the bracket fit. I told him I hadn't made it yet, and he pulled four little strips of aluminum out of a drawer (turned out to be 0.040" - just the right size!) about an inch wide and six inches long, and asked me if these would work to make the brackets with? I told him they would, and he said "ride safe young man" and shuffled off to the back of the store.
Anyone remember Fraggle Rock? The old guy named Doc, in the workshop? This fellow looked just like him.
I have all the pieces in place. I'm off all next week from work, as we're all closed for the week for the holiday, and I now have to try to remember how I made the first bracket...