Used to be a readily available steel cone seat insert for some of the larger truck wheels I used to see in the catalogs back when. Seems to me that Gorilla used to sell something like that. Maybe even Weld. Search up the lug nut companies and see what they offer.
If I remember, I thought they'd be perfect for converting an aluminum wheel to tapered nuts. They were thick, wide, and had a slight "shank" to them to center in the hole. So in essence, you would not even have to machine the face of the wheel if it was already flat. Only if you wanted to recess the thick washers so they weren't proud of the hub surface.
Either way though, I thought they'd be great.
Paul