Private Object Field in Django Admin

Question

I'm wondering what the best way to have an editable field (by admins) for some object in Django admin that can't be viewable on the public site. For example, a notes section for content authors with conventions or instructions that a front-end dev can't accidentally render in a template.

Something like:

class SomeModel(Model):
    internal_notes = CharField(max_length=1024, public=False)

So some object, o, can't be rendered in a public (non-admin) template:

{{ o.internal_notes }}