OK. In the meantime I've tried it out using the regular wall template. I'm almost there but not quite. Here's what I got so far...
A green wall added into the template. Part of the green expands beyond the squares although I tried my best to stay within the boxes.
and here's what it looks like when added as a wall in the editor...
Looking straight at it its alright but the angled views show a part that shouldn't be showing.