I´d like to revive this thread as I firmly believe you can still have a DDD difusser next year:
the only thing you have to make sure is: all opening s in the step or bottom surface must mot make visible any part of the car.
so this allows for a holes going through the car bottom up you could with this ,respecting suspension ,etc have say a tube ,round or elliptical going vertically through your sidepod .
But : why not open out this tube to the rear attaching a BIG divergent duct to it right to the back of the car? this would give an enormous pull in the tube ,lowering underbody pressure I could imagine .
To avoid feeding the tube from the top surface of the car ,a Gurney in front of the top opening of the duct would help create a low pressure zone there ...
heres just a small sketch of the idea:sideview of the slice through the car sidepod