I use Sketchup Make 2017 for 3D printing, and I would like to create a rectangular frame that represents the print volume (or at least the base) of my printer.

The problem is that I want this frame to be barely visible, and not something that interacts with my model. I tried putting the frame into its own layer:

But that didn't actually change anything. I can hide the layer, but that doesn't do me much good.

So I was wondering if there's some other way to do what I want. I tried creating guides, but I could figure out how to make sure that the guides are only line segments rather than infinitely-long lines. And even then, my model would interact with the guide.

Sometimes when I run CleanUp on a model, it will say that it reduced some edges (e.g. "Edges Reduced: 10"). But then I run CleanUp again, without touching or changing anything, and it will say the same exact thing -- "Edges Reduced: 10". It appears that CleanUp is notices something that needs to be fixed, but it doesn't actually fix it.

What causes this to happen?