One of the ideas I stumbled across on pinterest was a pin of a camera lens cap pouch, the original source was Crafty Staci. The instructions looked simple enough, I had fabric left over from my bench and I am always misplacing my lens caps (I even lost one in Kauai into the Pacific).
I modified the original plan a bit. I made it bigger since the lens cap for my 16-35L is 82mm. I also only used one fabric (I didn't feel like buying a yard just for this little bit). And more importantly I changed how the pouch is attached to the camera strap.
My finished pouch.
With my cap for my 50L in it.
The back, I didn't just make a spot to slip the strap through.
I made the pattern a bit longer and cut out two of them, then I made one big rectangle of them and sewed some more of the velcro on to it.
Then I wrap it around the strap and close the velcro. It's really secure.
On the strap the right way.
It also works on my rapid strap (which is what I am mainly using these days. I can't wait to really use it! I really hope this saves me from losing anymore caps!