need help with a port

[hawk.bd.usmc]

ok so i am getting ready to build a custom sub box for my chevy avalanche, its going to replace the factory midgate. im using 3/4 mdf and my measurements are 45'' wide, 15'' deep, and 20'' high. my question though is how big of a port i need because i have never built a sub box that had a port so i dont know how to do it. if someone could help[ me this that would be awesome. oh and here is a link for the subs im using, im using two of these HDC3.112 Copper Coil Subwoofer

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