Access to the page after logging in

I wonder if it is possible to block the entire gridbox page and give the client a preview only after logging in? I show clients production sites that are online so they can evaluate the work. I wouldn't want everyone to see this.
I know joomla has an ACL, but it is very complicated and based on the joomla login component. I would like the client to have a clean login form based only on gridbox after entering the website. After entering your login and password, a preview page would appear.

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