Need to use a pony wall and be able to set wall height

[MarnixGarner]

I have searched the help files and it does not cover my required situation as follows: I need to have a pony wall (used as outdoor fencing) - the bottom section of the wall is solid concrete as such with the top main part being wooden horizontal slats as you can see in the image. The overall height of this wall should be 1800mm but it cannot be set anywhere?

I can't recreate this wall using any other wall method. Incredibly frustrating! Appreciate a solution to this?