It’s really simple and you can buy super cheap used wireless routers daily on Craigslist and elsewhere.
They don’t need to be connected to anything other than a power source unless it’s the one you’re actually using.
The others don’t need internet access and their wireless security should be as tight as you can get it.
Some of them can be powered by 12vdc so you can keep one broadcasting in your car all the time.
Since they are almost always in residential and commercial spaces you can place them virtually anywhere.
As an ever-increasing number of smartphones report any wireless networks within range this will become even more
effective as the future progresses.
Here is a screenshot of four wireless router SSIDs so you can see how cool it looks!