Thanks for the answers.
I tried the "triangle" approach and that worked fine.

I want to try to make a wall with a sloping side. I found the slope options in the Modify screen of a wall. But it seems I can only make sloped walls that are wide at the bottom and narrow at the top.

I can make a wall like this:
|XX\
|XXX\
|XXXX\
|XXXXX\


But I want something like this:
|XXXXX ...