Good luck with figuring it out. I've tried doing some blocks that the rope will sit in and press them into the side of the neck but I just couldn't get them to stay still, so a bit of a fixture like you have done is neccessary.
There is a design that has been shared on here which was following the Korean Method (whatever that is) and it had two adjustable screws that can go onto the neck and tightened independently. I wish I was clever enough to make one like that.