You can take a look at the original horizontal layout file placed in the plugin directory: /views/horizonal.php
Simply copy it’s content into your new file.
Modify the layout like you require. You can use an image attached to the post for the icon.
Simply use the ‘get_attachment_link‘ function to retrieve it.
You also probably have to write some JS code to toggle the content visibility.
Following variables are available inside the layout file:
$this->pages – an array of pages of posts (WP_Post objects). Amount of pages is controlled by the ‘Visible items limit’ option.