Yes, you would need a patent to protect the custom fit. As kimura-mma said, it will be difficult. But, not impossible. You'll need a great patent attorney to properly draft the application. You should probably do a patent search to see if any other company has received a similar patent. You can do a search
here (the left hand section is for issued patents and the right hand is for patent applications). If someone was able to do something similar before, then you may have luck. But if you don't find anything, this probably means you'll have a harder time because, it seems to me, if it was possible, one of the larger companies would have done it already.