My son, Ethan, who is 12 likes to play Halo video game. So it was easy to use Halo as the costume. The entire costume is scratch build using paper and foam.
The helmet started with paper craft, pepakura. The helmet is then hardened with resin and bondo. The helmet is then finished with motorcycle visor. The rest of the costume is done using EVA foam, anti-fatigue foam. Everything is cut using templates and then hot glue the pieces like a jigsaw puzzle. Once all pieces are done, I painted it and weather it. Finally, I used nylon straps and Velcro tape to put everything together.
The weapon is also scratch built using PVC pipe and EVA foam. The entire built process can be followed in my YouTube channel: CoolCostume.
This is the third year I build my kids costumes from scratch. In previous years my kids get a lot of positive reaction from their friends and the public when they go trick or treating on Halloween. They usually get stopped so people can take photos of them. They will be wearing these suits at school and on Halloween this year.