wheel spacer/adapter, tire and wheel all need to be properly rated and designed. A "dished" wheel as for a dually will not work well if you just remove one of the dually wheels and leave one installed. The wheel needs to have the appropriate backspacing or offset for your application.
I would doubt that a chunk of billet steel or aluminum would fail, but I would certainly feel better about steel with some big fat grade 8 studs in there, maybe even an allen head bolt or torx bolt on the back side with some red locktite on them.
any machine shop could make a wheel adapter out of a "round" of steel. All they are doing is drilling holes. Cost and interest however may be other issues altogether..