Here are a few pictures of my Halo 4 Master Chief helmet. I'm not a stickler for 100% game accurate, so I have made a few changes. For the details on the visor, I used the negative of a two-part stencil. I then used a spray glass frosting spray (but you could use matte clear or something similar). I've just about got it all finished...
Fan-made Halo 4 Master Chief Helmet