example of how to override a charfilter which will be used for all charfields on a model in a filterset

charfilter_overriding.py

from django.db import models
from django.contrib.auth.models import User
from django_filters import CharFilter, FilterSet


class ICharFilter(CharFilter):
    def __init__(self, *args, **kwargs):
        super(ICharFilter, self).__init__(*args, **kwargs)
        self.lookup_type = 'icontains'


class MyFilterSet(FilterSet):
    filter_overrides = {
        models.CharField: {
            'filter_class': ICharFilter
        }
    }
    
    class Meta:
        model = User
        fields = ('first_name', 'last_name', 'username')