Dynamically create sidebars based upon page templates being used.

wordpress-custom-sidebar.php

<?php

/**
 * Dynamically register sidebar for pages that use specified template
 */
function showcase_register_page_sidebars() {

	// Find all pages that use template with dynamic sidebar
	// ideally this would be done in the admin only since it's creating an additional query that's not needed on most pages
	$query = new WP_Query( array(
		'meta_key' => '_wp_page_template',
		'meta_value' => 'page-templates/widgets.php',
		'post_type' => 'page',
		'nopaging' => true,
	) );

	while ( $query->have_posts() ) {

		$query->the_post();

		showcase_register_sidebar( showcase_sidebar_id(), get_the_title() );

	}

	wp_reset_postdata();

}

add_action( 'widgets_init', 'showcase_register_page_sidebars' );


/**
 * Get the sidebar id
 * can also be used in templates
 * eg: dynamic_sidebar( showcase_sidebar_id() )
 *
 * @param type $id
 * @return boolean
 */
function showcase_sidebar_id( $id = null ) {

	if ( ! $id ) {

		$id = get_the_ID();

	}

	if ( $id ) {

		return sprintf( 'showcase-%d-sidebar', get_the_ID() );

	}

	return false;

}


/**
 * global location for registering widgets
 *
 * @global type $wp_registered_sidebars
 * @param type $id
 * @param type $title
 */
function showcase_register_sidebar( $id = null, $title = '' ) {

	global $wp_registered_sidebars;

	$id = showcase_sidebar_id( $id );

	if ( ! isset( $wp_registered_sidebars[ $id ] ) ) {

		register_sidebar( array(
			'name' => sprintf( __( '%s Sidebar', 'showcase' ), $title ),
			'id' => $id,
			'description' => sprintf( __( 'Widgets for %s', 'showcase' ), $title ),
			'before_widget' => '<div id="%1$s" class="%2$s">',
			'after_widget' => '</div>',
			'before_title' => '<h3 class="widget-title">',
			'after_title' => '</h3>',
		) );

	}

}