Easy Guy Rope & Shock Cord Tighteners can be scaled to fit your cords.
I've attached 2 files; one that works well with 3mm shock cord, and one for 5mm cord.
I find them particularly useful when staking out my rain fly on my tent, or stringing up a clothesline. They also work very well with shock cord. Great for lashing items to your backpack or bike. Thread cord as shown in the pictures and pull the loose end to tighten. If the cord backs out see the last picture. This step is usually not needed but will increase the security.
I print them as shown in the 3d model pictures.