How to make meta box checkbox work in wordpress

I have this custom field with the name “en_proceso_class” and the value “en_proceso”. It was supposed to add a class to a div element in my post query.

// Get post meta that is already set
$custom_values = get_post_meta($post->ID, 'en_proceso_class', true);?>
<div class="postthumbnail <?php echo $custom_values; ?>">

But the problem is that if you are on a new post and the custom name “en_proceso_class” is there but the value is empty, I don’t want the client to add anything to the class since it’s already styled. I thought it’s best to convert this into a checkbox. It would be something like “Please check if you want the post to be in process” which will add the class to the post. I did research and again was confused by all those researching… Text field were simple but checkboxes were complicated

Here’s the code in functions.php that are half working. It works in that it adds class, sort off, but the checkbox isn’t working by unchecking and checking the box everytime you update. Additionally, when I make a new post, the class is added even when I made sure that I didn’t check the box when I published the post. Could you check what’s wrong with the code.

add_action( 'add_meta_boxes', 'cd_meta_box_add' );
function cd_meta_box_add()
{
    add_meta_box( 'my-meta-box-id', 'My First Meta Box', 'cd_meta_box_cb', 'post', 'normal', 'high' );
}

function cd_meta_box_cb()
{
    // $post is already set, and contains an object: the WordPress post
    global $post;
    $values = get_post_custom( $post->ID );
    $text = isset( $values['my_meta_box_text'] ) ? $values['my_meta_box_text'] : '';
    $check = isset( $values['en_proceso'] ) ? esc_attr( $values['en_proceso'] ) : '';

    // We'll use this nonce field later on when saving.
    wp_nonce_field( 'my_meta_box_nonce', 'meta_box_nonce' );
    ?>
    <p>
        <input type="checkbox" id="en_proceso_class" name="en_proceso_class" <?php checked( $check, 'on' ); ?> />
        <label for="en_proceso_class">Please check if this is in process</label>
    </p>
    <?php
}

add_action( 'save_post', 'cd_meta_box_save' );
function cd_meta_box_save( $post_id )
{
    // Bail if we're doing an auto save
    if( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) return;

    // if our nonce isn't there, or we can't verify it, bail
    if( !isset( $_POST['meta_box_nonce'] ) || !wp_verify_nonce( $_POST['meta_box_nonce'], 'my_meta_box_nonce' ) ) return;

    // if our current user can't edit this post, bail
    if( !current_user_can( 'edit_post' ) ) return;

    // now we can actually save the data
    $allowed = array(
        'a' => array( // on allow a tags
            'href' => array() // and those anchors can only have href attribute
        )
    );

    // This is purely my personal preference for saving check-boxes
    $chk = isset( $_POST['en_proceso_class'] ) && $_POST['my_meta_box_select'] ? 'on' : 'en_proceso';
    update_post_meta( $post_id, 'en_proceso_class', $chk );
}

By the way, I got that from this tutsplus tutorial, https://code.tutsplus.com/tutorials/how-to-create-custom-wordpress-writemeta-boxes–wp-20336.