public function sidebar_select_options()
 {
     global $wp_registered_sidebars;
     foreach ($wp_registered_sidebars as $key => $sidebar) {
         printf('<option value="%1$s"%3$s>%2$s</option>', $key, $sidebar['name'], selected(Custom_Sidebars::get_sidebar(), $key));
     }
 }
<label for="custom-sidebar"><?php 
_e('Register custom sidebar for this post?', 'custom-sidebars');
?>
</label>
<input type="checkbox" name="custom-sidebar" value="1" <?php 
checked(Custom_Sidebars_Details::has_custom_sidebar(), 1);
?>
 />
<label for="custom-sidebar-select"><?php 
_e('Use the following sidebar for this page:', 'custom-sidebars');
?>
</label>
<select name="custom-sidebar-select">
	<option value=""><?php 
_e('Default Sidebar', 'custom-sidebars');
?>
</option>
	<option value="none" <?php 
selected(Custom_Sidebars::get_sidebar(), 'none');
?>
><?php 
/* translators: as in 'No sidebar' */
_e('None', 'custom-sidebars');
?>
</option>
	<?php 
$metabox->sidebar_select_options();
?>
</select>