I would like to create said effect, but using a prop, plexi, and good lighting. You can place your prop (even an fcg) behind an enclosed area, and use slanted plexi to capture the illuminated shadow of your creation and make it appear to be suspended in mid air. I am sure that the rest of you are familiar with this concept, if you have any ideas, please let them fly. This concept was very effective in Disney's Haunted Mansion, as was the little guy sitting next to you when you passed he mirrors. Bring back memories?

Screamhaunt

I was going to use this effect last year on my fcg but ran out of time. Instead of plexi, which is large and can get expensive not to mention hard to store, use that plastic sheeting that you use to cover windows in the winter. Just build a frame attach the plastic sheet and use a hair dryer to stretch it tight. They sell the large sheets for sliding glass doors. I was going to use this for a whole room instead of a small section.

Oh, don't forget false perspective. You can make a 1' ghost and have it fairly close to the viewer but in the right lighting it will look like a larger ghost much farther away. Disney also used this effect at the exit of the ride. The ghost bride that talks to you at the end is actualy only two foot tall.