Apply Custom Fields Only to Top Level Projects

Currently, when you make a custom field, the only three options all involve applying the custom field into descending projects.  Would love an option to only apply the custom field to that specific project's level. We have a project structure where we want to be able to "tag" certain projects underneath another project without copying the top level project's custom fields down.  

E.g. We have "A" Projects, and we need them to be projects to allow for reporting.  But there are B and C sub-projects that need to rest inside A projects.  The custom fields we assign to A type projects are not relevant to B and C type projects, but as soon as we tag B and C type projects inside an A project, the custom fields pop up in list view for the B and C projects.