Step 1) Create a new temporary Design
Create a design and called it Coming Soon or something similar, and attached this design to your single coming soon / under construction page. (On your page, click on Settings icon and then choose your coming soon template under the Design field.

Step 2) Lock down your current Design (using PHP method)
Then your current website design, the default Design template for the rest of your website, click the Source icon and then the PHP tab and add the following code:

<?php

// disable the website until we are ready to launch website.
// only homepage 'coming soon' page is available for non admins.
if (!\Components\Settings\Administrators::currentIsAdministrator())
{
	\Framework\HTTP\Redirection::redirect('/');
}

?>

This will redirect all non-logged in Administrators to the homepage, which should be your Under Construction page.

Note, if your Under Construction / Coming soon page is is not the homepage, '/', then replace the code from '/' to your page name, such as '/coming-soon/' or similar.

Hope this helps