Display box in sidebar of custom post type

It’s really easy, it’s called meta box and you can create one with the following code.

/**
 * Register meta box(es).
 */
function plugin_prefix_register_meta_boxes() {
    add_meta_box( 'meta-box-id', __( 'My Meta Box', 'textdomain' ), 'plugin_prefix_my_display_callback', 'custom_post_type' );
}
add_action( 'add_meta_boxes', 'plugin_prefix_register_meta_boxes' );

/**
 * Meta box display callback.
 *
 * @param WP_Post $post Current post object.
 */
function plugin_prefix_my_display_callback( $post ) {
    // Display code/markup goes here. Don't forget to include nonces!
}

/**
 * Save meta box content.
 *
 * @param int $post_id Post ID
 */
function plugin_prefix_save_meta_box( $post_id ) {
    // Save logic goes here. Don't forget to include nonce checks!
}
add_action( 'save_post', 'plugin_prefix_save_meta_box' );