Good things come in narrow packages

[Sumpter250]

As for the backdrop - I think getting the perspective right (vanishing point, horizon line, etc.) is the trickiest part. How realistic you want it to look is semantics by comparison.
Galen

Steve, you have an interesting problem there. Even with the 24" depth of the my modules, the viewer, at "altitude" can't be fooled by visual trickery. He only has to move just a bit and the perspective is all wrong for what had looked good just inches to the left or right.
In your case, it looks like you have even less "foreground depth", and forced perspective becomes plausible, only from very specific viewing points.
Perhaps, in this case, "nothing", is actually a better way to go, or at the most, a very subtle matte painting to fill the emptiness, that has very little sharp detail in it. ( clouds at "height", and " green puffiness " A.k.a. " hazy distance" close to the "ground". )
I actually chose to simply leave the backdrop ( "sky board" ) light blue. The motion in the foreground keeps the attention there, and not "in the distance".
Hope it helps the "thinking process",
Pete