The yellow arrow points to a leather cord tied similar to a poacher’s knot to have an end loop.
The red arrow are ends of an elastic string which passes through a hole (one for each end) in the leather.
I need to tie the elastic string ends in such a way to secure both the leather cord, and on itself so that when the elastic is pulled, it will not stretch. Also it should be hard but not impossible to untie.
Also it should not be too bulky.
It’s a standard figure 8 knot, but the end of the rope doubles back through the eight. This leaves the end of the 8 at the top of the knot & would look similar to this: