I would probably not go down the path of cnc'ing a block of "ally"..
While it sounds great "ally" does not float!! So if for some reason it comes out of the case the whole thing sinks!! At least with wood you have a chance the plate/rudder can be retrieved.
Even though its old read the chapter in Frank Bethwaite's book.

For those classes which have a no carbon rule.. Very thin veneer or ultrathin ply is the natural carbon substitute.. One can steam veneer and epoxy together to make a side of plate.. then put in the backbone stringer epoxy the side together and glass.. should be very strong and light