Breaking News: I might be a bit of an idiot. I've just done careful measurements with a caliper. During these measurements, I realized that the ID of the hood isn't even threaded. There are just parallel rings lightly engraved around the ID. I made a bad assumption.

The OD of the front of the W-1 hood is 99.3mm, so I probably won't even find a good aftermarket solution. As LJSLATER suggested, I'll likely just find a good 82mm plastic cap. The one that came on the lens doesn't seem to fit the hood, but if I go to my LCS and try a few, I can probably find one that works.

Thanks for your replies!