If someone made a survey and checked to see what is the one piece of equipment you loose most, I am willing to place dollars for pennies that the answer would be lens caps. Those things just keep getting lost. I guess this why you have so many lens cap holders solutions out there.
Here is a nice idea by Flickr user RawSniper1 that uses nothing but 2 pieces of Lego, has a really shallow footprint and will save your cap.
It is probably one of those super simple ideas that make you smack your head and say how come I did not think about it before.
You would need a drill, some glue and a bit of wire to make this, but it is totally worth it if just for the innovation of using Lego in your process.
P.S. if you want something a bit more fancy, you can check Benny Johansson’s lens cap holder which is probably the cutest lens cap holder in the world.