Interesting. I would suggest going with the standard GDg tuning using strings ADg. I guess the best way to figure out the gauges would be to refer to a string pack for 12 string guitar. What are you going to use for the cone? This seems to be the hard part when you try to use something else than a real commercial cone. Most builders here make the mistake of using something way too sturdy like a pot lid. Real cones are quite flexible and delicate. The cone needs to support the downward pressure of all six strings, but at the same time freely move up and down, when you strum the strings. String tension too tight or too loose is a problem. It would be a shame to go through the effort of making an octave instrument only to have a flat tone. Those strings need to vibrate. Thanks for sharing. Enjoy.