I have had my two Odyessey PC680 batteries laying flat for 150 hours of operation now, no problems at all. Now only that, but they just happen to be a nice fit between the ribs under the floor of the baggage area. They are supported by the ribs, not the belly skin. I made my baggage floors removable, but it is a bit of a chore removing all those screws. Fortunatley those batteries don't need to be looked at very often.
Randy C