WooCommerce - Make checkout fields readonly

<?php
// WooCommerce - Make checkout fields readonly
add_action( 'woocommerce_checkout_fields', 'readonly_billing_last_name', 10, 1 );
function readonly_billing_last_name( $checkout_fields ) {
  $checkout_fields['billing']['billing_first_name']['custom_attributes'] = array( 'readonly' => 'readonly' );
  $checkout_fields['billing']['billing_last_name']['custom_attributes'] = array( 'readonly' => 'readonly' );
  return $checkout_fields;
}