There's a freebie that does such. In the mean time, you can also enclose your scene in a cylinder and keep the camera angle above the floor. If you use a primitive, make sure the shape has adequate roundness (say 50 segments) and subdivide to your heart's content. In Reality, set it to matte white and away you go.

I'll check my runtime for the name of that prop, but it was free, that much I do recall.

crimsonworx.com; free ebooks and previews

I've bowed down to facebook: https://www.facebook.com/crimsonworx