Hiding class namespace from plugin's extensions

I'm developing a plugin using this boilerplate and I designed it to be extensible. The boilerplate supports namespacing and uses it and namespaces are bind to folder structure. The extensible part is about loading a class from the extension and I want to require extension developers to:

1. check the existance of the class loader (namespace: Plugin_Name\Inc\Admin)
2. extend their class from a base class (namespace: Plugin_Name\Inc\Admin)

I'm looking for the easiest way to fulfill the above so that they won't get involved with my plugin's namespaces.