All Classified forums require a Site Supporter subscription. It is only $25 per year and you can buy and sell whatever you like on the forum. To become a Site Supporter, go to Your Profile and select "Paid Subscriptions."
I'd make it out of MDF or fiberglass if you can, metal is not very good for mounting speakers in if you're concerned about sound quality...especially a larger full range speaker like a 6x9. It resonates an awful lot.