The first picture are over exposed ... the second looks to have the natural color.
Now you need a bit of radio tuning string, few centimeters will do, so to be tight up with the holster, and become impossible to loosing it.
![Wink ;)](https://www.eevblog.com/forum/Smileys/default/wink.gif)
Few knots will work as anchor, from the inside of the holster.
and about the cap it self, a tiny hole
in one of the sides will be enough to pass the nylon string, and do few knots too.
If you dislike the color of the radio tuning string, an black permanent marker will do the rest of the miracle.