Make a custom field in admin post.php read only

Instead of making the custom field read-only, you can hide a custom field by prefixing the name of the custom field with an underscore.

e.g _custom_field_name

You can hide the field by disabling the input boxes using replacing CUSTOM_FIELD_NAME with the name of your custom field.

jQuery(document).ready(function () {
    metaName = jQuery("#postcustom input[value="CUSTOM_FIELD_NAME"]").attr("id");
    metaValue = metaName.replace("key", "value");

    metaName.prop('disabled', true);
    metaValue.prop('disabled', true);
})

You would need to put the jQuery code in a js file in your Parent or Child theme and enqueue it on the post.php page using:

function add_admin_scripts( $hook ) {
    if ( $hook == 'post-new.php' || $hook == 'post.php' ) {
        wp_enqueue_script(  'myscript', get_stylesheet_directory_uri().'/js/myscript.js' );
    }
}
add_action( 'admin_enqueue_scripts', 'add_admin_scripts', 10, 1 );