Shuv I was thinking if you need 3'x16' you should add whatever thickness the wood on the existing
signs is to the .040 before you have it cut. Say ½" extra for example.
(I can get my supplier to cut my aluminum for a small fee)
The sheets are 4'x8' so you will have leftovers.
The have someone with a brake bend it on the top edge so it makes a lip that would go over the top of the existing
sign. That way water would not be getting under the new face when it rains. As for attaching it, I would use both VHB tape and self-tapping screws. I had a
sign buckle once using just silicone.
You could make little white circles out of vinyl to cover the screw heads.
(although .040 does come in several nice colors if you don't want a white background)
Hey it's just a thought and may not be practical. Also I am not a carpenter or engineer.
Love....Jill