If you can finish this as per the drawing it will be awesome.
I agree with Rom, the cage is completely unecessary and weakens the design. If you carefully trowel and level the top of your concrete to which the metal pier bolts down to, then the top of the metal pier will be surprisingly level anyway and then perfectly rigid
Just cut an access hole in the side of the metal column so you can access the centre bolt for the mount head.
Mike