I have had two adapters water-cut to their basic shape in 20mm steel (actually, they measure slightly thinner than 20mm) and have drilled and milled the first one according to my CAD model and drawing. It does seem to be a perfect fit. Now I have to do the same for the other one and then give them some sort of surface treatment. Galvanizing and painting perhaps?
For the record, the material for the adapters is steel with a yield stress rating of 355MPa. As far as I can remember, my last analysis gave a max von mises stress of 200MPa. And yes, steel is denser than aluminium and my adapters are heavier because of it. But there are other considerations as well. Besides, why worry about the small extra weight from the adapters being made from steel instead of aluminium when I'm adding huge cast-iron rotors anyway.