Here’s the dimensions on the blank-center wheel cover from SOCAL:
The recess is “exactly” 75mm or about 3″ give-or-take
I cut the back “fingers” off of these Center caps (available on ebay or at MOSS) and here’s how they would fit. If I use these, I’ll turn a wooden plug to fit the cap, epoxy the plug into the cap, and then wood screw it to the wheel cover.
A center logo would have to be AT LEAST 49mm to cover the three mounting holes in the wheel cover.
Stay tuned to see the results of my “replace the plastic wheel covers” quest.