@Keet, are there some additional settings needed if you wanted to use a simple solid color as you suggested? When I do that instead of the transparent texture approach the rendering engine will cast shadows and thus make the color delete afterwards a pain.

Awww, thank you for the solution. I would have never known this! I'll have to import it into Affinity Photo to remove the black, but it gets the job done.

I'm trying to render a floor plan, but have the ground/space outside my floor plan render as transparent. Is it possible to do this natively from SH3D?

Here's a sample image online of what I'd like to mimic (the outside shadows around image are nice but not in scope of this inquiry):

https ...