Django: Refer to User objects in fixture for another model

Question

To avoid the XY problem, I will describe my problem and my attempted solution separately.

My problem

I have an Appointment model which I want to write fixtures for. There is a one-to-many mapping from the Django User model to the Appointment model (every appointment has only one client, but a client can have multiple appointments).

I know I can hard code the primary key for the Users in my appointment fixtures, but would be a brittle solution. Hard coding the foreign keys also compromises the readability of my fixtures.

My attempted solution

I looked at this question, which has an answer linking to Django documentation for natural keys. However, I ran into a problem attempting to use this approach.

I believed I could create a proxy model for the User model, so I could create a custom Manager for natural keys. Here is my proxy model:

class UserProxy(User):
    objects = UserManager  # The UserManager class's code is omitted
    class Meta:
        proxy = True
        unique_together = [['username']]

(If the UserManager class's code is relevant, please let me know in a comment, and I will add it.)

However, when I run manage.py makemigrations, I get this error:

ERRORS:
accounts.UserProxy: (models.E016) 'unique_together' refers to field 'username' which is not local to model 'UserProxy'.
    HINT: This issue may be caused by multi-table inheritance.

I understand this error message: the username field belongs to the User model, not to the UserProxy model.

My questions

1. Is my proxy model approach a good approach? If it is not, what should I do instead?
2. If my proxy model approach is a good approach, how can I correctly refer to the User model's username field in my unique_together list?