...but its really HEAVY. My ex-studio partner had a piece of Plexi that was 4'x6' and 2" thick, he used it for exactly what you have in mind. It was so heavy it took four strong guys just to stand it up on one end. How many assistants do you have? Of course, he needed it thick to support the body builders he used to shoot, but I'm thinking you'll need at least 1" thick to support a model of reasonable size. I'm not aware of any other way to do what you want, unless you made the supports out of clear plexi too. You'd still probably get some kind of shadow or indication that the support is there, but it might not be too hard to PS away.
Regards,
Andy Pearlman
Andy Pearlman Studio