All done. Used the charts from this blog, and a general gauge x kid-head measurement for the size. Yarn is Mirasol "Tupa," and I used size 3 needles.
The chart parts were done back and forth, intarsia style, because I haven't mastered intarsia in the round, and this worked out best, although the "A" needs some help. I joined and did the top in the round, then seamed the bottom part. Then I undid and redid the seam because I discovered that it's generally better to seam black wool in bright sun versus a dim evening house.
The resident Pratchett fan is pleased, and warm*, and that's all that matters.
*Hey, it's summer in the Bay Area. It's chilly!