Hey, I’m doing this exact thing on my Mark VI Silver Timeline build! I’m for sure not going to be able to explain this perfectly, but here was roughly what I did.
I loaded the file into Meshmixer, generated face groups and extruded the face groups at the places I wanted to make the splits out to make separate objects I could use to subtract from the original model. I then performed a Boolean subtract operation, leaving me with the back half of the model separated from a deformed front half of the model. I saved the back half as a new object and loaded the original file back up.
I did this process again, grabbing the next face group over, this left me with the front half of the model and a deformed back half. I saved the front half as a new file.
I loaded both the front and back halves into 3D builder, added two cylinder shapes I split in half at the seam line of the model, added holes for magnets, merged them with the front and back halves, and now I have a removable faceplate. I also added guiding wholes for a peg system but the magnets aligned the faceplate perfectly.
Frankly Built explains a similar process that I tweaked in this video:
Link
After work I’ll take a look at Moe’s file and see if I can find a good place to break it up and if so I could try and get you something you could work with.
Edit: Doesn’t seem like there’s a good place to split it this way for Moe’s file unfortunately, the helmet is too smooth and there’s not a good face to grab and extrude. A couple creative plane cuts could get you most of the way there though,
Good luck!