How to change field Model attributes when inheriting from abstract models

In my code, I use a lot of abstract Model classes. When I inherit from them, I often need to change some of the attributes of specific fields. I am trying to implement a solution where those attributes are defined as class variables which can then be overridden upon instantiation.

Here is a simple example of what I am trying to do:

Let's say we have an abstract model class as such:

class AbstractThing(models.Model):
    MAX_LENGTH = 1000
    IS_UNIQUE = True
    name = models.CharField(max_length=MAX_LENGTH, unique=IS_UNIQUE)

    class Meta:
        abstract = True

What I am hoping to do is this:

class RealThing(AbstractThing):
    IS_UNIQUE = False
    MAX_LENGTH = 200

However it does not work. The resulting migration file looks like this:

migrations.CreateModel(
    name='RealThing',
    fields=[
        ('id', models.BigAutoField(
            auto_created=True,
            primary_key=True,
            serialize=False,
            verbose_name='ID')),
        ('name', models.CharField(
            max_length=1000,
            unique=True)
         ),
    ],
    options={
        'abstract': False,
    },
),

Is this just not possible or am I doing something incorrect? Any ideas would be really appreciated.

class RealThing(AbstractThing):
    IS_UNIQUE = False
    MAX_LENGTH = 200

    name = models.CharField(max_length=MAX_LENGTH, unique=IS_UNIQUE)  # override it

class AbstractMeta(ModelBase):
    def __new__(cls, name, bases, attrs):
        print((name, bases, attrs))
        attrs['name'] = models.CharField(
            max_length=attrs.get('MAX_LENGTH', 1000),
            unique=attrs.get('IS_UNIQUE', True),
        )
        return super().__new__(cls, name, bases, attrs)


class AbstractThing(models.Model, metaclass=AbstractMeta):

    class Meta:
        abstract = True


class Foo(AbstractThing):
    MAX_LENGTH = 100


class Bar(AbstractThing):
    MAX_LENGTH = 200