Thanks for the reply!
True, it all goes back to Pythagoras :D
This last photo wasn't so helpful, probably, as I was trying out complicated stuff!
But, basically, I think you can do any width this way. Here's a more primitive build to show you what I'm thinking. You'll note that the wall is fully connected on the base and to the top plate.
Therefore, it can be whatever length you want it to be!