The pattern was a total hack. I figured out what the height and diameter of my head were and then cut a rectangle of fabric a little bit larger than that. I stitched it across the top, then pinned it down the back and along the front and back of the top seam to make a more-or-less rounded head. These seams then got sewn and trimmed.
Next I added a zipper along the back.
Once that was in place, I folded the fabric underneath the chin and sewed an indentation there, then put a hem in along the bottom. Voila! I'll probably add grommets at the bottom back so that it can be padlocked into place, but it works just fine without it. I could probably tweak the shape a bit, but it doesn't seem necessary.
In the side-view picture, the funny lump at the back of my head is because I have my hair pulled into a bun.