Open your project with the public IMyInterface
interface shown above.
Go to File > Export Template
.
Follow the steps (make sure you choose item template rather than Project template) and save it under a name of your choice.
When you restart visual studio and have a project open it will be available from the add item dialog. You will be able to set the name of the interface from the dialog just like you can with the other items.
Note that this does not overwrite the default interface template installed with Visual Studio so you can still just as easily make a private interface when required.