i went w/ a side by side pipe. inlet w/ a dip tube to the bottom of the box and the outlet just entering the box. i debated sloping the top slightly to make the vent area the highest point, but i figure if the tube is at the top, and the top fills up with gas, it would go out the tube anyway.
the way i understand the tube in tube design....the heat exchange is when the cold air comes in, it warms from going thru the warm tubes, so as to not lower the inside temp of the box. i decided not to bother with this, as a 2' length of pipe probably won't transfer heat to the air fast enough. unless there was a real heat exchanger, or the air moved reaaaaally slowly.
wonder if there is an inexpensive hydrogen gas meter i could stick in there to see if it's working or not.