Here is the perfect solution, a fully customizable 3d printable screw in lens cap you can print on your home machine.
- Cap Diameter
- Cap Pitch
- Cap Tolerance
- Cap Thick
- Cap Label Position
- Cap Name
Here is the files you need and Customizer: http://www.thingiverse.com/thing:414556
Detailed build guide is here:
The project was developed by Prof. Hank Dietz
There is even a company which makes something similar, but since now you can make it at home, maybe they will run out of business.