This is very helpful for me - thanks Bob.
The rubber around the central slingshot triangle thing (with the targets) - can you stretch that somehow to fit the shape you want or do you have to create all the layers of walls by hand?
You have to create all of the layers for each new different shape by moving control points and adding control points. It's not as hard as you would think. You can left click and hold the left click down and highlight or select entire areas and copy and paste them.
For a completely new rubber shape, I select a similar existing rubber, just copy and paste one layer and then reshape that layer and then copy it and paste it on top of itself 5 more times while adjusting the height and color of the walls for the rubber in this style.

It's easier than it sounds and practice does help, usually only a couple of minutes for something like a newly shaped plastic, rubber and posts.
Here's another pic of my example layout, it definitely needs something, maybe the recycle bin.