I was looking back to see who had suggested the fishtape....and it was you!
AND NOW I understand why you said yours broke when you got excited and stuffed it in your pouch.....you removed the plastic coating which is the large diameter part of the fishtape and what helps it be strong and stiff!!!!!
See I've been trying to break it when it's inside the rope and I can't......and even when it's outside the rope it only really deform when putting alot of pressure at a 180° bend.....
But I can definitely Crack the very thin inner fiberglass when it's pulled out!
Fiberglass and carbon are very weak in shear especially at small diameters....
But when it's inside the flexible plastic sheath its not going to break! And with the rope around it providing radius enlargement protection, I have tried every possible position in the saddle even in the cold and it's held up great!
You and
@atwoodnative should try making a tether with the whole thing and not just the fiberglass part! Trust me the 3/16" will easily fit inside your rope....and it'll be much stiffer!