Possibly you could just use an off-the-shelf hotspot solution, like:
http://www.hotspotsystem.com/ - (used personally, I have a reseller account with them) - you can buy pre-configured routers from them directly, or you can put their custom firmware on a number of supported routers. I’ve set this up on a TP-Link TL-WR1043ND V3 and it works fine.
http://smb.purple.ai/ - (not used but researched before) - buy their small device that connects to your network, pretty much plug & play I believe.