Making an addon for this would not take 5 minutes. If it would, why don't you do it? You would have to make a way for people to define new zones, store those zones, and make it user-friendly. You're asking for help, please don't act entitled to it.
There are addons on scriptfodder.com for a few bucks that will let you define zones. I am pretty sure some of them will allow you prevent people from exiting those zones, in which case you can just cover your map with one. But don't just take my word for it, I wouldn't want you to buy an addon that didn't work for you; Ask the creators of those addons if you're unsure.