You can see an example here:
StatBanner
is a native class defined in components
directory:
In your case, that would be api.renderInOutlet("above-main-container", YourClass)
I don’t think you can do it in the HEAD file. You should split your code into multiple files.
I encourage you to use Discourse Theme CLI as it will be much easier to develop a Theme component!
Is your Theme Component public?