Personally, I just make sure the valve is closed tight and place it upright in the cargo bin down below. There are no openings other than the door but, frankly, how is this any different than putting it in a trunk or the back of an SUV to take it and have it filled? I think the reason the on-board tank is in an open area is because it's pretty much installed and forgotten. Your portable tank comes out when you land and is seen often.
Hopefully, I won't get taken to task for doing it wrong. Then again, If I've been causing a danger, I suppose I'd like to know.