The spinlock housing on the Boxster are slotted a certain way. You either have the spinlocks on the wrong sides or turned the wrong way. Pull one out and look at it closely then at the housing on the Boxster.
Ask me how I know...
... Let's just say it was quite the 'duh' moment after spending an hour fighting with the bolts and taking them in and out several times before finally pulling one of the spinlocks out and inspecting it.
Make sure you use the plastic bag method to keep from dropping one of the bolts into the car or you may end up with a permanent rattle.
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