I created this document as a "first attempt" sort of thing more than a year ago.

I've returned to LucidChart and found it sitting there. It looks something like what's posted below. There is a red heavy-bordered area. It's non-selectable. It's exactly what I want to reproduce but I don't know how to do it. Everything I try involves putting something on the canvas which is selectable. I just want to color an area of the document like this.