Hey AceNat,I'm currently building a suit for my girlfriend and have all of it completed except for the robotic arm. I've read through the entire thread, but couldn't find mention of which robo bicep and elbow you ended up using. Was it the one from the Official Halo Reach thread that you...
I had originally cut where you marked in order to fit on my girl's head, but it still proved too hard to slide on, so I ended up cutting off another inch from the back portion. Great pepping!
Good luck! I'm also working on a Noble 2 suit for my girlfriend, and I'm aiming for Halloween too! I found AceNat's thread to be particularly useful, and probably wouldn't have done it if not for her. Keep us updated!