A little scene developed by chance as I was working out the outfitting and surfacing of some of my characters. When I got to the point of treating it as a scene, I realized the HDR I was using was too low-res. I set it aside until I came across a similar background while googling images. I made a number of subtle changes to the background while compositing in post, zooming in on the part I wanted. If I can remember where I got it, I'll add a link.
Edit: I stepped back to the final render with the original HDRE, since the lack of shadows on the other background kept bothering me, and I am unable to render a correction at present.