Hello there, I apologize for the lack of response.
The collar was a challenge to overcome because all the helmets clip through the collar in the game. Originally; I was going to use a marine helmet that was just about too small for my own head, but it rubbed against the collar and restricted all movement. I was determined to keep the collar as it was one of my favorite parts of the costume, so I scrapped the marine helmet and started toying around with other helmets that were flat-jawed and shallow. I had a failed MK V{B} helmet I printed that was too small for my spartan (and just about too small for myself) that I tried on with my armor, and the two paired together perfectly. My head movement is still restricted; I can move with some side-to-side motion, but I can't look up or down without tilting my entire body or stressing the collar (which is not a big deal; my collar is in two halves that have magnets and elastic, so it snaps back into place after I pull my head forward). That being said, I have a longer than average neck that helps with the distance between the helmet and the collar. An ODST helmet may be a viable option if you pay close attention to your scaling, but I've never tried.
If you have any additional questions, I would be more than happy to answer them to the best of my ability.
