How do I fix a 3D Box with 3D Billboard on another 3D Box?

I like to fix a 3D Box with 3D Billboard on another 3D Box.

How do I…

Making an object by combining 2 3D Boxes.
(One has 3D Billboard)

What is the expected result


(The bottom of the 3D Billboard 3D Box sticks on the 3D Box)

What is the actual result


(Off-center)

(Half is sunk, black and wither line is the horizontal middle line)