For the back of the costume which is the monster, I made the torso out of Styrofoam. The mask head has a plastic milk jug for stability. The head is attached to the Styrofoam by long thin nail from the rubber mask stuck into the Styrofoam. There is a slit cut midway from the black robe where the torso of the child sticks out. The styrofoam is taped to the child’s body. I put a piece of cardboard on the front and back so the tape would not bite in.
The fake legs are pantyhose stuffed with rolled up newspaper and were pinned to the pajama top. I bought the head and black robe from Zellers for apx 25.00 dollars so this was not a huge investment.
You need to make sure the fake legs are not too long as the person will trip over them when walking. My son was the hit of our large townhouse complex for trick or treat.