I would just scuplt it out of clay mold it then make a pull out of latex. Attach it to your bodysuit then if you have a Predator mask take some fabric. Attach the fabric to the bottom of the mask then the other end to the neck seal...so when you turn your head in the mask it wont pull the neck seal the fabric leaves enough room to turn your head. I suppose if you needed a foam neck seal you could do it but i think latex would do just fine.
Here's something to ponder. I saw this on a Japanese site. Don't know where the cast came from. But if my memory doesn't fail me his neck seal does come under his chin too. Not that far. Check out that wrap around the back.
This was easy and cheap to do. I made mine with windshild wiper tubing,3/8 diameter. I cut the pieces to lenth, glued them with Household Goop and used black felt for a backing so it wouldn't grab against the painted neck but could still hide where the edges met. it also was adorned with brass segments for some accents instead of painting them on. They fit perfectly and still alllow for the head to move freely. I matched it up to a DB headpiece. really complimentd the greens in the paint scheme nice. This version doesn't spread out when it meets the chest but looks great either way. I can add a picture just tell me who to send it to for posting.
I have made many different kinds. My sugestion is to take Tubing with size you want or you can use Electrical Power Cord. Glue the pieces together. Make a Slush Mould out of Plaster and Pour until your hearts content out of latex.