Two of my friends and I do a local Halloween contest on a dinner / party cruise boat every year. Each year we make our own costumes. So far we have one first place award (for these Homemade Lego Men Halloween Costumes) and two second place awards. We decided on the Legos because we wanted to see if we could pull it off.
The heads were cut from 12″ sona tubes with insulation foam at the top and bottom (for the rounded edges). The faces were painted on using a paint marker. The mouths were cut out and backed with black window screen to hide our eyes. The bodies were cut from cardboard leaf containers. The bodies were then covered with colored duck tape and accessories were painted on.
Good luck and have fun!