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I use a BookArc with my 2012 MBP, and as I am looking at it right now, I don't see it fitting in there with a Speck hardshell. It does rock a bit from side to side, the BookArc does not fit as tight as you might think. But to make it fit in there with a hard shell, you'd probably have to push harder than you'd like.

Also, I hear those Specks act as an insulator for the MBP, might be heating your computer up more than you realize. Then again, I don't have one, so I couldn't say for sure.
 
I use my 2010 13 inch with a black speck hard shell on it in a book arc every day. Use the rubber gasket that is meant for the 15 inch MBP (it comes with a 13 and 15 gasket) and you'll be fine.
 
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