Needed to build the costume on my son, or something the same size as my son.

Since I didn't think he would stand still for 40 hours, we made a mannequin of him.

Stuck some PVC pipes in and filled a pair of his too small PJ's with moisture cure urethane foam, otherwise known as 'Great Stuff'.

To make something big like this actually cure, you need to spritz it with water as you are filling.  Normally moisture cure urethane foam will only cure up to a thickness of about an inch.  Soak the PJ's in water and hang them from a hanger.  Start from the bottom, and every inch of thickness of foam give it a good spray with water.

As it was just about cured but still a little soft (about 30 minutes) I beat it back into shape.

Then we tore/cut off the fabric and carved it back down to match his size.  

Sorry about consuming your wig head mommy.

Wear a chemical respirator when working with large amounts of urethane foam.  The stuff doesn't smell, but you can become sensitized to the isocyanates they release, which will make you feel like you have the flu the next time you come in contact with the stuff.  Trust me. 

Some arms where added, and the entire thing was coated in acrylic (shipping) tape to prevent the contact cement from sticking if it soaked through.

Then on went the black spandex full body suit I had sewn earlier to fit him with a zipper down the back.  I wanted it to fit exactly right so I made one from a workout shirt, some women's tights, and a spandex tube for the head from an earlier project.  

If the spandex suit fits too tightly, when you remove it from the model it will contract and the neoprene will tear off the spandex.

Sadly I didn't get any photos or video of my son creeping around the house in it like a ninja.