WordPress Gutenberg - Add a locked template to a post type

gutenberg-locked-template.php

<?php
add_action('init', function() {
    // For which posts type
    $post_type_object = get_post_type_object( 'post' );

    // Which allowed blocks
    $post_type_object->template = [
		['core/heading', ['level' => '5', 'content' => 'Default value']],
		['core/paragraph'],
		['core/image'],
    ];

    // Makes it impossible to move posts
	$post_type_object->template_lock = 'all';
});

Thanks! <3