On my Legacy, I also have a tiny internal gas tank on my 4hp Tohatsu, which is good for about an hour or so. In case I need some more gas, I have a small 1.5 gal tank that's sitting on the cockpit sole at the back, held with a bungee cord strung from one side of the stainless arch to the other side (hooks just below the cleats). It's out in the open, which is a must for storing gasoline (don't store it down below).
I never really gave it too much thought about a better place for the tank, but so far, it's small enough and out of the way enough not to be a problem. I don't even think about it anymore.
I take the spare tank home with me at the end of the day, and use it on my lawnmower, so the gas in there is always fresh. Just to be sure, I always use Marine Sta-Bil in the gas to prevent problems in the motor.