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.