The first version uses 2 mm nylon weed trimmer line and the string or cord patch loop is taped to the line using insulation tape.
This is hard to wind on to the line and cord without some bulges . When you have some resistance during your pull the cord can pull out of the insulation wrap.
To improve this I used some heatshrink tubing. The cord was fed through the heatshrink and the two loose ends held while the nylon line was pushed thorough the tubing. Take out your heatgun apply heat and you are done.
The second improved version was made from a long length of heatshrink tubing . Take some thin copper wire, run it through the tubing and make a flat hook, attach to cord loop and pull through. Apply heat to the end of the heatshrink (about 150 mm) to secure the loop, at the other end of the tubing thread the cord through a toggle coat button or dowel with hole in centre .