How and where to define a partial helper module
Your current solution is fine, in my opinion. The underscore naming-convention used on view-partials doesn't apply for modules, that's why you got the uninitialized constant-error. If you name it ApplicationHelper::ApplicationContainer it has to be in a file named application_container.rb

What you have in your code now is a module you can include in other Helper-modules– just as you would a partial in your views.