Modular Keychain

Do you have multiple set of keys in your keychain? Do you want to bring with you your pen drive? Do you wish to carry anything you want with your keys? This is the modular keychain. You can build millions of adaptors to bring with your keys and attach it to the main element, the attachment mechanism is very simple, but reliable, and easy to build, as simple as a T extrusion.
You will need a long M4 screw and a nut, or (I think that is the best solution), one M4 screw plus a long female threaded tube.

Please, be careful, this is a WIP, any comments are welcome. When I can finish the main element, I want to build a better USB adaptor and a slimmer keychain to carry another set, as the work keys.