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1st & foremost any 11" wide wheel will be a non-stop pain to try & run on a Boxster.
8.5" on the front will not be practical either & the 19" diameter will have enough tradeoffs to consider reconsidering 18" max.
Back to your question if you must use a spacer it must be hubcentric meaning the center must be a snug fit on the center of the brake rotor, & the spacer must have it's own center bore for the wheel to fit on & support it's weight, NOT the lug bolts.
You & your son will be glad later if you return the wheels now!
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