It's going to be really difficult to make anything that's light enough for an electric toy helicopter to keep it afloat. Even a few grams can make or break the lifting ability of those little guys.
Instead of coated foam, vacu-formed plastic might be the way to go. That way you're starting with a glossy surface, and after you form it, it's hollow and super light. If you use clear plastic, you can paint the inside so it stays high-gloss.
Additionally, it takes full-time concentration and a lot of practice to make those little copters fly even in an unmodified state. The guys at the mall kiosks make it look easy because they're doing it for their entire shift every day. To remove the skill element (or lack thereof) you might tie your prop to your costume with a bit of fishing line and put the copter on full power, so it reaches max height and stays there... like a tethered zeppelin.
Got any reference pictures so we can advise you better?
Last edited by nathancarter : 01-11-2013 at 04:18 PM.