Do the Monster Slapdash
Tape this to your back. It'll look science-y enough that people won't risk looking stupid by asking you what your costume is.
Kludge (klooj, n.): a clumsy, inelegant, improvised solution to an engineering problem. Kludge-O-Ween (klooj-oh-ween, n.): a makeshift, half-assed solution to Halloween. When it's too late to worry about finding the perfect costume, it's time to realize that ANYTHING can be a costume.