Wow! 12", sounds as though you were lucky if that was the only damage. My skylights are 14 x 14 so I am relating this as totally wiping out a skylight opening, and you didnt say there was any other roof damage so I am assuming none. If it is the skylight I am envisioning they are available throught Amazon. A quick search showed a bunch of options for under $100. Pretty easy change out. Maybe also take this opportunity to put on vent cover or upgrade to the max air vent fan. Good luck to you, as I said it looks like a pretty easy replacement.