I gave a friend of mine a set of dimensions for a set of wheel spacers and he created an Auto-CAD file for me.
I purchased the 6061-T6 Aluminum plate and took it and the Auto-CAD file to a local shop that does Water-Jet cutting and came home with a beautiful set of spacers.
Next, I drilled 4 small holes in the spacer and drilled corresponding pilot holes in the wheel and screwed the spacer into the inside of the wheel.
Why did I do this? Because fixing the spacer inside the hub of the wheel effectively makes the hub thicker and look like a single piece of material.

Paul