For the last couple of years, my son has asked me to make him a robot Halloween costume. This year he even drew a sketch for me and he insisted on the traditional cardboard box body. Since I knew it wouldn't be very comfortable or practical, I always talked him into something else.
I finally gave in this year since I could see that he really wanted it. I found the best ideas and tips through this site and ended up creating this "Candybot" costume. The most frustrating part was creating the head. I figured a box head would limit his vision and he would not want to keep it on. I saw the bleach bottle idea on here, but THOUGHT I could come up with something else. Through trial and error using many different materials, nothing seemed to fit as comfortably as the bleach bottle and I ended up going with that in the end just added a chin strap to keep it even more secure (the "eyes" were a little heavy and tended to pull it forward without the strap).
By the way, EVERY house we went to said my son won the prize for best costume. Many asked us to wait while they went to get their cameras, most gave extra candy, and the kids really loved it because of how cool the lights looked after dark. I also made a more comfortable back-up costume using silver bubble insulation(kinda like the windshield sun reflectors). He wore that after the box became uncomfortable. I used Velcro closures and carried it in a bag for a quick change while trick-or-treating. We got tons of compliments on that one too!
Good luck - it's lots of fun getting creative with odds and ends you find around the house, and even more fun when you can involve the little ones with you. Also teaches a great recycling lesson!