A hatch is not a bad idea. It would be very simple to just make a section of the top sheeting in front of the canopy into a hatch. But tanks are inexpensive and it never hurts to have spares, so I'd just buy an 8oz. And MIkeB makes a good point about more consistent running.

You didn't say what brand tank you have, but I know Sullivan, DuBro, Great Planes and Hayes tanks all have a third hole on the stopperand end cap, or nipple for a third fueling line. You just have to punch the rubber stopper or the third nipple (did I type that?) the rest of the way through.