Rear storage box on Duetto

Hi everyone,
Having acquired a lovely new driveaway awning for our 2000 Duetto, we're now wrestling with where to carry it when en route! The shower room is the obvious choice, but then it needs manhandling out if (when!) the loo is called for on our journeys.
I was toying with the idea of a rear storage box, but it feels like a heavy weight to be hanging off the rear door.
Does any member have any experience or tips  on fitting such a contraption on a a twin rear door van?
All advice gratefully received!