Here is a video i made a while back. I still use this knot on all customers strings and haven't gotten any complaints.
There are a few key factors to tying an adjustable knot that will still move.
1) size of material.
I know most people say you want something larger than what you're using, but I haven't found that to be true. When I serve a string, I am typically using PowerGrip .032 as serving and using PowerGrip .021 for my nocking points. Now, you don't want to use something so small that it slides in the groves completely between the serving, but you do want it to be smaller imo.
2) Stiffness of material.
The stiffer the material, the more easily it will be able to move. I can use a softer .021 serving like sx and not move it nearly as easy as I can move the Power Grip. It's due to the stiffness of the material. My all time favorite material to use is the power grip. A good cheap compromise is BCY's nocking point material. It is relatively stiff, but about 1/3-1/4 the price of the power grip.
For anything material related I would contact Chad Weaver (LBR) on here. I would bet you anything he has some short spools that could be sold at a good price.
If you need anything, let me know.
God Bless,
Daniel