Well, it ain't autocad, or even one of those newfangled 3D printers... this is what passes for rapid prototyping in my world.
Step 2: make template out of cardboard. Line up caliper mounting holes and (as best you can) determine location of axle hole.
Step 3: do the same thing with wood. Now you begin to really pay attention to the shape of the piece, the exact location of mounting holes, and where the axle will go. Wood allows some test fitting, including the spacer sleeve that will be press-fit and welded into place once everything is set. The key thing here is that the hub of the stock wheel is a different diameter than that of the Carrozeria wheels, so I did have to remove the wheel from the bike to double-check everything.
Step 4: start on the aluminum piece. It's 6061 3/8" alloy plate. This is the same temper as the sleeve that will work as a spacer for the rear axle. Drill holes, chamfer, cut to shape. Grind, file, sand, and grind some more.