A 285/60R17 is 30.5" in diameter.
Last week I found a site that had dozens of hard tire covers, but for some reason the site wouldn't let me post a link. I kept getting "invalid proxy response."
Search google for "hard shell spare tire cover." The place I found was called Morris 4X4. They average about $300 but it looks like they might offer free shipping on some/all of them.
Maybe you can find something that will work. They do seem to be for narrower tires, though they offer up to 32" diameter covers. Unfortunately, you may have to resign yourself to the fact that what you want is so specific that it simply isn't made.
Putting 10-ply tires on half ton trucks since aught-four.