Here's the thread where I captured what I did...

Grand Prixs Of North America • View topic - Updating Car Audio -- a/d/s/, ZED Leviathan, DIYMA R12

Here's the current speaker...

DIYMA.com - Scientific Car Audio - Truth in Sound Quality - View Single Post - 2004 Grand Prix GTP -- Rookie Install

If I were you just go to Fi or DD or similar audio company and build a speaker box per their already established configurations. They typically offer a few build guides for ported and sealed.