Not sure what software you are using to create your scene but have you tried raising the whole booth above the origin or adding a transparent image below the booth in your software before importing into Shapespark.
I’m working with 3dsmax 2019 - but I need only the shadow underneath. So, its not a question of the software
Like to have the normal background and not putting evreything in a “shell” - a PNG with opacity in the non shadow area would be good. (No real shadow - shadow from pic). But a “shadowcatcher” would be also working