The key to making a costume for a wiggly three year old (or at least my wiggly three year old) is to make it flexible and as non-constrictive as possible. I did do some sewing for this costume but it's entirely possible to make it without sewing thanks to duct tape. I'll discuss each major part of the costume in its own step and talk about alternative ideas.
Step 1: 1: Assemble the helmet
The hardest part of the helmet is finding the right size box. If you can't find one at home, you could either construct a smaller box from a big one or go someplace where there are a lot of boxes, like ask at your local grocery store. This part of the costume my son liked the least and he was prone to taking it off. There was a light drizzle when we were out so I managed to convince him to keep the helmet on to keep his head dry.
After I cut the box to size and cut an opening (see my attempts to check the size on my model), I then covered it with foil. It was pretty easy to wrap and only needed a small piece of glue and tape to finish it. I then attached the "ears" which were small tin pans on the sides and an inverted pan with a pipe cleaner antenna on top. I did cut out notches on the sides to make clearance for my son's shoulders. All of the pans were attached to the foil by tape.
So, the bill of materials for my helmet are:
* 1 box
* 3 small tin bowls
* black pipe cleaner antenna (leftover from another Halloween project)
* tape, and lots of it (masking and duct)
* sharp cutting tool (a box cutter would have been idea but I had to settle for half of my take apart scissors)
Other options for helmets include:
* Use a gallon milk jug, sides cut out; spray paint it silver - this will probably fit your child's head more snugly
* Get a gray ski cap and attach pipe cleaner antenna and maybe some goofy ears