> So I was hoping for a solution that would operate on mesh

// an object with 8 corners (clockwise front ABCD) (Clockwise rear EFGH)
#macro Free_Box(A B C D E F G H)
mesh {
   triangle {A,B,C} //front
   triangle {C,D A}
   triangle {A,E,H} //left
   triangle {A,D,H}
   triangle {B,F,G} //right
   triangle {B,C,G}
   triangle {A,E,F} //top
   triangle {A,B,F}
   triangle {C,G,H} //bottom
   triangle {C,D,H}
   triangle {E,F,G} //back
   triangle {G,H,E}
   inside_vector <(A.x+G.x)/2,(A.y+G.y)/2,(A.z+G.z)/2>
}
#end

You might want to move the inside vector outside the box
to invert it, and it's up to you to keep the walls flat.

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