I made a complete Halo 3 Master Chief Halloween costume for my Son out of cardboard and fiberglass. I started with the helmet. Here are the steps I followed. A small action figure and images found on the internet were used as patterns. The proportions and details may not meet the scrutiny of many 'super fans' but I'm sure my ten year old will like it just fine.
He has two Spartan weapons, a Sniper Rifle and an M6G Magnum
I have only detailed the build of the helmet here.
October 27th 2009 - I have just added pictures of the new helmet, chest plate, should shields and shotgun I built to modify the Master Chief costume from last year into a Halo Scout for this year.