Here is a handy tool. http://www.diyaudioandvideo.com/Calculator/Box/
http://www.diyaudioandvideo.com/FAQ/Build/
Both are tools I have used for building enclosures or cabs. The site contains a wealth of tips for builders like us.
CHRIS, Charlottetown PEI Canada. Anytime you can repurpose, reuse, or recycle, everyone wins!