We finished this costume about an hour and a half before we had to leave to go trick or treating at my sister's house. It was a lot of work, but the results are awesome. I made the electronics, and my wife did the cardboard papercraft as well as the actual design. Of course, I helped with final assembly. There was lots of glue that had to be clamped and held.
We initially spent a lot of time searching for other robot costumes people have done, and one thing we noticed was that most people make the costume too wide, so the kid's arms are held out the whole time, which we didn't think would lead to the kid wearing it for long. We found a box that was almost exactly armpit width, so that was the foundation.
It's mostly made from cardboard painted with textured spray paint. It's incredible how good that paint looks. Here the kids are testing out the Candy Input slot. Candy goes into a box inside the costume, and it trips an electric eye on its way in that triggers some sounds and the analog gauge.
He actually wore the costume for about a half hour before he wanted to sit down. There isn't enough room for that so we took it off, and I couldn't believe how heavy it was with candy! He was a real trooper for wearing it for so long.
The sleeves are dryer vent tubing, stitched to the long sleeve shirt he has on under the costume. So when he took off the box, he still looked a little like a robot. Also, notice the copper painted crocs. He loves those, and wants us to paint other stuff in the house copper now. We told him we'll see
Here's a better view of the back and the Arduino controller. There's an Adafruit Wave Shield there that drives the sounds, and I soldered up the circuitry you see there as well. The Arduino has no problem looping sounds, reading input, driving the analog gauge with PWM, and chasing the LEDs there around the controller. When I was setting it up, it ran on a fresh battery for over two hours without any degradation.
Last but not least, here's a video of the electronics in action. The green LED thing is a Game of Life kit that is independent of the rest of the circuitry, just for fun. I overboosted the speech in Audacity to make it seem like it's louder.
More about the initial design can be seen here.