Make ModelMultipleChoiceField with FilteredSelectMultiple widget - read only (Python Django)

Question

In my project i use an intermediary model CustomizedUserGroups to keep relations between CustomizedUser and Group models:

# models.py

class CustomizedUser(AbstractUser):
    first_name = models.CharField(max_length=255, verbose_name='имя')
    last_name = models.CharField(max_length=255, verbose_name='фамилия')
    # other fields
    groups = models.ManyToManyField(Group, verbose_name='группы', related_name='users', through='CustomizedUserGroups')

Therefore in admin class (CustomizedUserAdmin) for CustomizedUser i can't let fieldsets contain 'groups' (otherwise The value of 'fieldsets[2][1]["fields"]' cannot include the ManyToManyField 'groups', because that field manually specifies a relationship model. will happen). So, i defined fieldsets manually (with no groups) and problem was solved.

# admin.py

class CustomizedUserAdmin(UserAdmin):
    fieldsets = ((None, {'fields': ('username',)}),
             (_('Personal info'), {'fields': ('first_name', 'last_name', 'patronymic', 'role', 'email')}))

It is intended, that value for groups field in model CustomizedUser is assigned automatically depending on other values from registration form fields, therefore when a user adds a new user via admin panel, there is no need to display groups field. But when a user wants to edit other user's data via admin panel, i want the group field to be shown to him in read only mode.

To solve this i created ExtendedUserChangeForm, where i defined a group_list field:

# forms.py

class ExtendedUserChangeForm(UserChangeForm):
    group_list = forms.ModelMultipleChoiceField(label='Группы пользователя', queryset=Group.objects.all(),
                                                required=False,
                                                widget=FilteredSelectMultiple('группы пользователя', False, attrs={
                                                    'disabled': True
                                                }))
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        username = self.initial['username']
        user = CustomizedUser.objects.get(username=username)
        if user.groups:
            self.fields["group_list"].initial = user.groups.all()

I also assigned this form as a form in CustomizedUserAdmin and made fieldsets dynamically include group_list, when ExtendedUserChangeForm is used. It solved the most part of the problem. When i try to edit other user via admin panel i really can see groups_list field which displays actual content of groups field for this user, but read only mode (which i activated via disabled=True in widget attrs) works only for the left half of the widget, and i can still click on its buttons, while i'd like every part of the widget to be unlickable. Please, look at the screenshot to see, what i mean. How can i do this, while continuing to use FilteredSelectMultiple widget?