Thanks David, that kinda works, but gives a jagged slant that does not fully cover full-width containers.
I ended up using a jagged SVG (just 2 lines of text actually) to make a much more dramatic edge. I put it into a Custom HTML block at the bottom of the container and gave it a negative margin-bottom to pull it down onto the bottom of the container, and 120% width to make it push over the container padding. Looks great.
Hope this helps someone else….