Managing Manual WordPress Maintenance (the functions.php file)

Hello there. For some bizarre reason, no Maintenance Mode plugin will work on my site/host -- the homepage always appears, although links to it lead to "Under Construction" pages.

So, I was very happy to come across your article, "A Quick and Thorough Guide to WordPress Maintenance Mode." (https://premium.wpmudev.org/blog/maintenance-mode/)

I am using Method 2 from the article, "Manual WordPress Maintenance," and it finally, finally works. Hurray!!!

My question is this: Is there a simple way I can create some kind of "On/Off Switch" for Maintenance Mode that would sit in my horizontal Admin Bar, so I don't have to mess with the functions.php file every time I want to turn Maintenance Mode off and on? Ideally, I'd like something as easy as a light switch.

I'm a newbie, so if you could be as specific as possible (i.e., Explain To Me Like I'm 5), I would be very grateful!

Thank you,

Mike M