Think finding a hard cover for an 18'' wheel is tough,,,I am trying to find a soft cover for my spare and am having no luck. Being that it is a 42'' IROK on a 16.5 steel wheel may have something to do with it.
Kidding aside you might find one in the quadratech book.
I really am looking for a tire cover for my spare. Someone told me a custom boat cover shop may be able to help but I don't know any truth in that.