When I welded up the frame for the lighthouse mountain and sign I welded in four inch and a half square tubes that stood out from the finished sign face. These were to fasten the lettering to when we routed it from Precision Board.
The lettering vectors looked like this. I combined them to form the outline of the sign.
Then I combined the shapes to form the outline for the back layer of the sign. I also added four small squares to accommodate 1" square tubing that would protrude out of the back of the sign. The square tubes would slip into the square tubes on the mountain sign face.
The inside layer of the sign would have a space routed into it to accommodate a 1" square tubing frame. This would give the lettering shapes stiffness and provide structural support. This framework would also line up with the tubing that will stick out of the rear of the sign.
Tomorrow I'll post some pictures as I weld up that frame and assemble the three layers of the sign.