I think you all are missing the point, there's no possible way to make a 5mm spacer that also extends the hub (like a hub centric spacer). The center hub that the wheels use to center themselves on extends at least 15mm on the Boxster. Anything short of a 15mm spacer will, by definition, not be a hub centric spacer.
Note the 3 tangs on the center of the hub in the attached picture. Those are the centering tabs the wheels use to center themselves. A spacer would have to be thicker than these tabs to require it's own hub center thus making it a "hub centric" spacer.
BTW, if you decide to mount a wheel that isn't machined with a 71.6mm hub center on the Boxster don't be surprised when the car vibrates the steering wheel out of your hands above 70 mph.
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