More vapor generally means more wattage. The more wattage you have, the lower the ohm coils you use to support more power. The Ipow 2 is a 12W device. It won't produce clouds, but it can produce decent ammount of vapor. It can go as low as 1 ohm.
so, my suggestion, is try a subtank nano with the 1.2 ohm coil similar to like ou2mamame suggested and turn up the wattage some.. try it at 10 watts and then step up if it's not producing enough.
That's not saying it'll produce the most vapor, since there is a multitude of other factors to consider. But I think it's a good starting point. The three subtank modes (standard/Mini/Nano) basically use the exact same coils, but with different tanks. The Epow 2 is 18.5 MM diameter. The subtank is 25mm, the subtank micro is 22 mm, and the nano is 18.5mm... so, for looks, I would choose the Nano. If you don't mind a larger tank on the end of your Epow2, then go with the mini. Just keep in mind it will be top heavy.