Copy the bogus page(s), to your Omega. You can do this many ways, depending on your operating system. If you are on a Unix system, you can write: scp /path/on/your/laptop/to/index.php root@192.168.x.x:/etc/www where index.php is the captive portal's landing page and 192.168.x.x the local IP address. Don't forget you can also connect to the Omega using a USB cable, which I find particularly useful.

Go back to lighttpd.conf and check what is the username and group name the lighttpd server has. The defaults one for me were http and www-data respectively: nano /etc/lighttpd/lighttpd.conf

Give lighttpd the appropriate permissions to the www folder. Navigate to the website's root folder: cd /etc/www/

Then change the ownership of the files in the folder: chown http:www-data *
Finally, assign the appropriate permissions, e.g. 644: chmod 644 *

And that was it! Now you have a captive portal on the Onion Omega, or a Physical Web Space as I would like to call it, that has the same SSID as the network you want to attack, will lead the users to the same page as the legitimate hotspot does and will log down the credentials of the ones that were not careful enough to notice that there is something wrong going on.

Easy huh? Well, that is why if you are using such networks you need to pay extra attention. If you want to avoid falling victim to such an attack, the most obvious give away is the non encrypted (https) connection to the landing page where you need to supply your credentials. Generally, you should NEVER submit critical information in plain text. Furthermore, depending on how well the legitimate website is replicated, there could be other smaller details that should make you suspicious. On the other hand, the companies providing such services, should deploy the appropriate measures in order to hinder such attacks. But more about those, on another article in the future. :-)

I noticed that with the above setup, if the Onion Omega does not manage to connect to a local network, then the WiFi hotspot will not be created. In other words, it will not work when you are at a random place outside. I will look more into the specifics, but a quick fix is to go to the /etc/config/wireless file and comment out the local network details.
First: nano /etc/config/wireless
Then comment out by adding "#" the relevant network details, where your Onion Omega used to connect to, in order to get access to the internet.