There has recently been an RPG source book released for the Firefly/Serenity universe. I haven't dug into it yet, but I'd be willing to bet good money that if there isn't a ship diagram in there, that one is or soon will be available.
Gaming is a good source of stuff like this for those subjects that haven't been properly researched and modeled. The only problem there is FINDING who has it and is willing to let it get loose!
See ya!
Jim