The Niewiadów company in Poland that made the Cadet
did mount the body the other way round when selling their trailers to Britain, to get the entry door on the left side.
The slope of the rear window is irrelevant in aerodynamic terms, but the curvature of the roof is beneficial. Putting the sloping window at the front might make it look
more aerodynamic, but wouldn't do anything to reduce drag behind most tow vehicles.