How to design Electronics devices ?

If you just want one or two and don't plan on selling them, the easiest way is to roll up your sleeves, break out the soldering iron and make it yourself! (Or find someone with a maker-mentality that would be willing to make it for you as a personal project). This is a project that is at the level of most hobbyists.  Make sure you start with a regulatory approved power supply for safety reasons, and then regulate out the power from there (one voltage regulator per connected device) and connect to the right connectors and cables.

(EDIT): Note that if you really want it to be as small and lightweight as a typical _travel_ adapter, that is not a task for an amateur. You are trying to supply a lot of power to charge that many devices, and will likely end up with something about the size of a laptop brick to handle the power requirements and safely dissipate the heat generated by the regulators.